Hmmm. Trying to resettle black Africans in Zimbabwe would have invariably ended up with somebody pissed off - either white Rhodesians driven off their land, or blacks for being forced to live on shitty land.
I don't believe that by 1980 a solution was possible. I think if you want a multi-racial Rhodesia you need to go to 1953 at the latest. Unite Northern and Southern Rhodesia, let Nyasaland go when the time comes, and go to majority rule eventually. Bring guys like Kaunda, Mugabe and Nkomo into the government in the late 1950s, and come to an agreement on allocating seats based on race for a while, with a set timelime to moving to majority rule.
In 1970, Kaunda and Mugabe and others would not have accepted this. In 1960, they probably would have.
Now, Northern Rhodesia (what is now Zambia) was then (and still is to an extent) sparsely populated. Allow whites to keep land there, but grant the majority of it to the blacks. Create a specific education program to teach educated black Africans to be real agribusinessmen and overall improve the education system. High copper prices in the 1970s, no costly UDI and Bush War and growing agricultural exports, along with the complete lack of debt at independence, would allow the growth of the education system to be swift. In 1960, education is somewhat lacking. By 1980, it most certainly wouldn't be.
Have everything go right, and by 1980, Rhodesia would have about 25 million residents, probably 500-600,000 of them white, a prosperous economy, the best living standards in Africa (except for maybe SA) and the nation as a real alternative to the apartheid state - which would almost certainly guarantee it lots of economic help from the EEC and USA.