I'm not familiar with the 54 and earlier car trannys, but if the case is all one piece, then no, you can't convert it. If it has a bolt on tailshaft housing, then maybe, but I've never heard of it being done. Regardless, you would need to buy a later 3spd tranny for the parts anyway, so the issue becomes rather moot. Get the newer tranny and find out for yourself.
You will probably be using the 55 and up tranny anyway, which means changing the bellhousing, clutch fork, etc... also, since the tranny pattern changed when they went to an open driveline. You could also go with the T5 if you wanted to, once you've changed out that bellhousing. Some crossmember and floorpan work will be needed also.
The '74 Nova will work. Just measure it and compare to your original, and also keep in mind that the backset of your rims needs to be considered when choosing a rearend width.
Also be aware of where the axle centerline is.
Emergency brakes, shocks, etc... will change...
As for the driveshaft, It may be possible to find one that fits without modifications, but I don't know of one offhand. Get the tranny and rearend in first, then make the measurements and see. If you can't find one that fits, then get one that has the right ends, but is too long, then all you need to do is get it shortened and balanced.
