yeah thats the only way i could see doing it. Thats a LOT of work for a questionable fix, we are talking about the crank here afterall.... New cases or crank is def. the way to go. imo.I have heard of people repairing worn main bearing housings with bronze bushes , i don't see why it wouldn't work to get the imperial bearings into metric housings, alternatively you could could do the same thing with aluminium and carefully tig weld the joint and clean it up with a boring head in a mill.
Probabily just easier to sell the cases you have and buy another set.