i'm doing almost the same thing with my 79, and I was able to find everything except the gearbox for free...
A lot of people actually take out perfectly good manual's and put in autos!
A friend of a friend gave me a pedalset, bellcrank, bellhousing, flywheel, clutch, and pressure plate in exchange for my th350, and then I found a nice T10.
Basically, right now I am waiting only to get into a garage so I can work on it.
The only thing I don't have is the shift boot to fit into my console, but that's purely cosmetic and will wait until I get the rest of my interior taken care of.
We looked at the crossmember from mine compared to a crossmember from a manual firebird, and they looked identical, so at least for my gen car that isn't supposed to be an issue, and we measured the t10 with the bellhousing as being about a half inch shorter than the th350, and there is about an inch and a half of yoke sticking into the gearbox when the rear end is hanging off the car, so that shouldn't be a problem. Again, don't know about your year in that respect, plus I don't have a muncie to measure.