If you are going to keep the C4, you will have to shorten the tubes on the double hump to get it to fit in the frame brackets. If you swap to a T-5 or an AOD, you will either have to cut and re-weld the rear bracket, or modify the crossmember to get everything to line up. The reason is, the brackets 0n '79-'81 cars are only 2 1/2" in width, while the '82 and up brackets are 4 1/4" in width.
I had to go through this with my '79 to mount an AOD and dual exhaust.