with the tranny options did you mean th400 and 700r4? i've never heard of the 70064 being a gm guy (i'm guessing a typo happened).
how much power are you looking to put to the ground? the 700r4 can be built to survive quite alot of power reliably, but at its max it can stand just a little bit more than the th400 could stock. if high power is what your aiming for (say 400ft/lbs of torque or more) then i would go with the th400 and add an overdrive unit from either advance adapters or ranger if you want it, or go with a 4l80e with a stand alone computer (or a computer for both the motor and the tranny like what comes in the 90's era gm vehicles).
if you went with a manual tranny then either an sm465 or an nv4500 would be the best options.
what drivetrain you go with depends on what your plans are for the vehicle. will it be a daily driver that will see highway speeds? will it be trails/rocks only? will it see baja type offroad? will it need to do all of these or a combination of these?
if it wont be driven on the highway than a tranny without overdrive would be just fine. cheaper, fewer things to go wrong. th400 auto or sm465 manual.
if it will be a daily driver or will see baja type offroad, an overdrive would be a good idea (or if it does everything). 4L80e or nv4500 would be the ideal choices here, or a th400 or sm465 with an overdrive added, then finally a 700r4.
