[Coley]

The sbc is a great motor to build and work on...there are almost no limitations on directions or performance...and it is a very economical engine to fix up, build or modify.
Are you doing this for the sake of economics? or interest? or?
...as mentioned the 'crate engines' are pretty cheap relative to what you might spend in building one....or at least competitive and a lot faster and easier to get done.

As for the transmission...you would be better served to get past the TH350/400 option and move up to the 4 speed automatics like the 700R series or the 4L60 transmissions. You will get better mileage and performance out of them.

Coley