I mated a mid-'80s Camaro T5 with an S-10 tailshaft before installing it behind the stock 307 in my '68.
Full story
here
Here's the story of the tailshaft swap necessary to get the shifter in the right place.
I would not hesitate to put a T5 behind an early smallblock, especially in our trucks. Just don't drive it like an idiot and you should be good to go; I loved my setup.
If you decide to go with a Tremec, check out American Powertrain. They have kits and all the stuff you'll need and are very helpful.
Hope this helps!