Don't see an unknown 20 year old Jeff Goldblum being cast as Golum if you're having filming start in 1972. There's also no reason to cast Mark Hamill.
I also don't see that kind of box office for a 1970's sword and fantasy movie in the 1970's. It wouldn't be unique enough to bring out enough to generate a billion and a half dollars in the 70's.
Peter Jackson's success was helped by state of the art CGI special effects that didn't exist in this time period.
Also there are too many Americans in the cast. Kubrick is in Britain and would be surrounded by top quality British actors. He may hire an American as lead for the benefit of the US audience but the rest would be British to keep costs down and delivery more believable Middle Earth characters.
Again, I said there were definitely things I would improve should I rework the concept. However, one of the things I'd probably keep is Jack Nicholson as Aragorn, seeing as Kubrick was anxious to work with the actor by the 1970's (he reportedly wanted to cast him as Napoleon).
Regarding the special effects, I believe Kubrick could use practical effects effectively, especially given the budget. Remember, many of the city-scapes in the OTL trilogy were rendered using miniature models, not CGI.
But you're right, even if the whole trilogy was extremely successful, it probably wouldn't make as I originally envisioned. Maybe altogether around $750 million.