rolland, there may be a solution. I am currently trying to get CEN part number USPS13 to fit through the FF diffs. The part comes from one of the on-road cars I think and it is big enough to accept Savage drive shafts. It is also the exact same design as stock FF diff cups. The only small snags that I have are that the need to recess further into the diff casing and also, they require a bit of trimming of the upper hinge pin mount and the very bottom of the shock towers. This is to give clearance for free movement of the diff cup. All I can say is that it is a VERY close match. I am also looking at the possibility of one of the on-road diff gear housings may fit, thus allowing the diff cups to recess fully.
EDIT...Don't know why I didn't check on this before. It would appear that the part I have is discontinued so availability is sketchy at best. Gutted really, these things are really close to solving the diff cup problem. Ah well back to the drawing board.[]