Well I've only done 1 6.1 5spd in a 68, and wasn't excited about the drivetrain angle but it did work out ok.It is larger than the 727 currently in the car. The 8HP70 fits in the 66 and up B bodies with a little minor trimming to some of the bosses on the transmission case (nothing structural). Only minor trimming of the floor brace above the transmission tail housing is necessary. I'd like know conclusively that it will fit the early cars just as well. Or to know conclusively that it won't, so that I can pursue other options. We are problem solvers after all. I'm just trying to flesh out as many problems as possible before the build begins. This ensures a smoother process once the build starts and keeps me from having a half assembled project cluttering up the shop while we sort through the inevitable problems on the fly. We are known for getting projects done quickly and with a high level of quality, and that is due to proper research and planning. It is something we are quite proud of.
I'm thinking there will be Trans tunnel issues, but hope you figure it out