The NV4500 is a very big transmission, it will withstand over 500 ft. lbs. torque which is overkill for a GMC V6. I would suggest using a smaller, lighter gearbox, say from a Chevy pickup the Getrag I have in my 1992 GMC 1/2 ton is a very good smooth shifting transmission and would work out great for your truck with no changes to floor or crossmember needed and it also should be cheaper than the NV4500.
