More info is needed.
Which motor? 270? 336? other?
what size pickup? 1/2 ton? 3/4 ton? 1 ton?
What rear gears? GMC 1/2 tons typically had a 3.08:1 rear ratio. This ratio is great for economy, but horrible for an overdrive transmission.
Other transmissions are a near drop in such as a saginaw 4 speed, muncie 4 speed, warner t10, etc. Most of these conversions will require cutting a hole in the floor.
Most importantly, how are you planning to use your truck?
John
