Hey hemi hunter, I'm not sure how many versions of that 3 speed they made, but I don't know that it would be strong enough for that 350. Ofcourse that would depend on what you are doing with this truck.
My buddy restored a 77 chevy stepside and when he went to get an estimate on getting the 3 speed rebuilt, the tranny guy told him he would probably break it. He was running a very mild 400. He ended up just converting it to a turbo 350.

good luck, josh