I cannot give you Japan as the world's Nr 1 economic power (without reducing other countries ASB style), but I can give you Japan as the worlds #1 industrial power, even with a 1946 POD. Note that the earlier independence and scientific planning only servers to bring this around earlier, not make it possible in itself.
The Scientific Planning ideas of Edwards Dewing reach the Japanese industry much earlier, and Deming gains the ear of Japans bureaucracy, industry and the LDP in 1950, when Japan gets independence two years ahead of OTL due to the Korean War. The Japanese quickly develop a 'Toyota Way', and by the mid 1950's are maybe 5 years ahead of OTL. However, the newly established MITI (Ministry of Internal Trade and Industry, although it was more of a bureaucracy), was run by even less politically inclined, pragmatic people than OTL. These were planning very far ahead, and they also actively sought new techniques of scientific management to apply to the Japanese economy.
In the late fifties, they come into contact with the Technocracy movement of the USA. They spent several years investigating the movement's ideas and scientific contributions, and found that if they were correct, the Japanese economy would stall due to overproduction and overinvestment, possibly in less than 50 years. Although the causes would be different than what caused the 1929 depression in the states (due to Japan being an export country), the same solutions would suffice.
Thus, they, based on Tech Inc's plan, developed a detailed plan for the transition to a post-scarcity economy once the technical preconditions were in place. They also made sure to safeguard this transition against political intervention, by preparing a massive education campaign in secrecy.
In the 1970's, Japanese products were the rage all over the west (probably 5-10 years ahead of OTL now), and Japan's industry was larger than ever before. Due to this the 1979 oil crisis occurred in 1975, because of increased demand. This had also been, while not predicted, at least warned against by a certain Hubbert, who the MITI has also investigated (since he was a Technocrat). Thus, an action plan was already ready, and Japanese suffered less due to it. This also proved to Japan's politicians that the scientific planning of MITI was very accurate.
About this time politicians became a bit scared of MITI, as they acquired more and more influence, and sought to limit it. However, MITI has built propaganda channels of their own, and declared the diet's move to limit scientific planning a stab at Japans heart for selfish reasons. They also implied that politics might not actually be the best way to run a country.
Then, in the mid 80's, the Japanese economy went belly-up, only this time it was anticipated, and MITI had started their educational campaign 6 months earlier. Their set date for the collapse was only wrong by 7 months.
The MITI plan, now distributed to every home and fed on TV regularly, called for a total conscription of labor for the industry (that was keeping Japan alive), fixed salaries and prices, and nationalization of all industry. The various CEO's and economist, and especially the politicians, cried communism, but the police weren't listening to them anymore, and the Japanese people, being very good at following official directives (just watch those earthquake drills), did as MITI suggested. During the next few years the MITI gradually took control over more government functions, and the fixed salaries were evened out as trains and apartment rent were made free. The most work was done in the industry however, as different corporations industrial sectors were integrated, and a uniform resource accounting system was introduced. Industrial production was normalized at the levels of consumption, and exports were gradually reduced to the levels needed for Japan to maintain an import of materials.
On April the first, 1989, the new economic system having been designed by MITI was introduced. It was a form of energy accounting, based on the original idea by Technocracy Inc. Short story: The total production of the nation is measured in energy, this is then divided evenly across all adults, and given as an energy certificate, allowing that person to requisition his/her share of the production. In 1989, this was only slightly over the pre-crisis average salary, but the industry was rapidly improved and by 1994, the living standard was equal to about $100,000 (2009USD)/Year and person.
At the same time, due to Japans exclusive scientific administration (with the exception of subjective issues, now solved by direct democracy), Japanese industry was without doubt the most efficient on the planet, and perhaps with the exception of military hardware, also the most advanced. By 1994 the Japanese industrial capacity (although not output, resource conservation was the tune of the day) was now approaching 50% of the US output. The Japanese Technate was a fact.
The rest of the world had initially not reacted tremendously, since the MITI's actions was seen bye some as a form of "Japanese New Deal", and those who believed the Japanese were really turning to communism thought it was a good idea, since many Americans were scared to be economically eclipsed. By 1989 however, there was no longer any doubt that Japan had gone red, and Washington promptly demanded that Japan reinstate her political system. When this did not happen most news in the west started talking of a coup in Tokyo. Misinformation was rampant and many sources claimed the involvement of the SDF.
The US forces in Japan had been confined to base ever since April. Now, the new Japanese government was kindly asking the US to consider withdrawing. The US forces kept paying their bills (in the Yen that Japan still kept for foreign trade) however, and so the Japanese kept delivering food and supplies.
In late 1989, after the system had been in place for 6 months, the new Japanese administration (it refused to call itself a government), announced its intentions to the rest of the world:
1. Japan is now a Technocracy, operating a Technate instead of a monetary system.
2. The Yen is now exclusively for foreign trade. The amount available will be fixed. All unprinted yen must be held in the Japanese foreign trade bank accounts.
3. Japan will, in exchange for raw materials, provide the best assistance and technology she can to foreign countries.
4. Tourism will be restricted temporarily, but eventually the gates will open again. Immigration is allowed on a case-by-case basis.
5. Japan will continue to fund their part of the UN, and provide humanitarian aid.
So, now we have a Japan that could potentially pass the US in industrial capacity in the early 2010's or so, should they want to. US industrial capacity might decrease however, as new and more efficient Japanese products might compete successfully against US industry, turning the US more and more into a financial empire, with massive out-sourcing.
The reactions of other nations are unlikely to be pretty though. I'd imagine that third-world countries might want to trade with the now more generous deals offered, but would the US try to cut off ties? After all, they have experience with Technocracy and now what it will do to the political system (and not even politicians like losing their jobs). The Soviet Union, for as long as it is around, might trade or not. On one hand, they are supposed to be communists, on the other hand, the Soviet political establishment is just as afraid of losing control to engineers and scientists as the American one.
Thought, anyone?