I'm assuming you are going to install the SBC yourself ( motor mounts, exhaust ect.... ). Why not go with an automatic? In my CJ-5, me & Daddy threw in a 350 mated to a TH400 to the stock Dana 20. Yes one adapter, but we found it used and in real great shape. The Dana 300 is a better TC than the D18 or D20. Both the TH350 and TH400 are great trannys.