Yeah, it fits. . . Have seen it in both a 5A and three different 5Cs. (5Cs were available new with two different OEM powertrains: 8V71/4-spd manual or 6V71/HT-740 automataics.)
You have to fabricate a new mount for the turbo, the ones I've seen have been mounted on the driver's side of the engine. As Bob mentioned, the squirrel cages are mounted lower in the 5s than in the later MCIs, so you have to work around that. Not impossible, just takes some imagineering.
Plus, of course, you've got to upgrade the cooling system to handle the higher HP. Not sure if the OEM 8V92 radiators for the MC-9s will fit a 5A, but that's what you should consider, as well as larger squirrel cages and a smaller diameter blower pulley.NC Bob -
Fred Hobe's correct regarding the MT-643 being the only automatic transmission that will fit with the 8V71 engine.
It's a smaller unit than the 740, doesn't have quite the torque capability, but is still a good unit. There's one model in the MT series tho that doesn't have a lock-up torque converter, so you'd want to avoid that one. Sorry, I don't remember the actual model number, perhaps TomC might have the info in his references. The MTs were also used a lot in skoolies, another source for used.
As I said before - all it takes is money!
FWIW & HTH. . .