The snowball effect and a trans question?

[69C10Atl]

Well I bought my 69 c10 lwb fleet almost 2 years ago and for the most part it has just sat in my dad's garage. Now that my dad has caught the bug for my project it gone from simply making it a daily driver to a much more indepth project. We were just going to dismantle the front, (from the firewall forward) but now he's talking about making a hoist so we can get the cab off. hince the snowball effect. Anyway, the trucks a three on the tree and I want to replace the three speed with a four, can ya'll offer any help on what I should be looking for? I do have access to several junk yards in the area, so are there any particular vehicles to look for? Thanks