Once it is possible retrofit M3 parts into the sedan, i thought that both chassis was almost the same. With your statements, it seems that the sedan is more roomy to receive the front axles transmission and diff.
I would like to see instead, a E30 with your engine/transmission. In my mind the S54 engine doesn't suit good in the E9x body: old engine to newer body. Besides that, the E30 is a much lighter body and makes it easier to achieve your goals.