I'm guessing from what you've posted that you've got a KCC (or KVA) type car?
Try a post on the GTD owners forum as (a long time ago) GTD used to make weld on sub frames to do exactly what your looking at for early KVA cars. Some of the guys in the UK still have this set up and it works well, if anyone with one of these cars is reading this over to you.....
Otherwise try giving Frank Catt a call, I recon' the easiest way to go would be to buy a pair of complete rear corners (uprights, hubs, wishbones, shocks etc) and graft them on, this avoids the changing this bit to go with that bit to then change this other bit to fit that bit.... and round and round we go (yep, we've all been there)!
