C20 eh? Thats pretty damned sweet!
Hmmm, unfortunately im not 100% knowledgeable with the trucks back then and requirements on the K-member and all. Hell if thats how its set up back then, never had the chance to work on one.
Well you know, 454s are popular. That would be my choice, again i have no idea how and if that will work on a C20.
You should read this, help you decide on the three different generations of BB.
3 Gens of BB
This is a toughy, ill keep reading see what i can dig up for transmissions.