You'll need some sort of of adapter plate, the V8 is an easier conversion as the LT77 to V8 bellhousing was a standard item. You may be able to do something with the TR7 2 litre bellhousing but I don't recall ever seeing an LT77 conversion on a 4 cylinder P6 before.
As already mentioned, suffix C or D (or an F if you can find one) will be the strongest, but any would do for your needs. Budget for a rebuild if you buy a secondhand unrebuilt box, you're unlikely to find one that doesn't need some work.