a T5 may not be what you want behind a 350 V8 because of torque considerations, it may not be much help, but for some...
http://www.motivegear.com/tech_info/tranny_guide/tranny_guide.htmljust as a side line look at SM465, NV3500 & 4500
But then again it ALL depends on what it is you want this truck for .. use wise...........
towing, race, show, lack of $$, general driving, hauling ???
visit inliners international
http://www.inliners.org/ ... tech, tranny section
what the gear ratio of the rear end ???