Any automatic transmission from a Chevy V8 equipped vehicle used from the mid 1960's to the early 90s will be a bolt-on fit to the 230/250/292 six, with nothing but a flywheel change and possibly a different starter. The bellhousing shape is the same on the sixes and 8s (even the Mercruiser 4 cylinder) from 1963 and up. I'd suggest a simple TH 350 3 speed. They're rugged, easy to rebuild if necessary, and pretty plentiful. The TH400 consumes too much power internally- - - -it was designed for heavy vehicles and high torque V8s- - - -Buick , Olds, Cadillac, big block Chevy, etc. A 400 is also about 50 pounds heavier than a 350. RUN- - - - -do not walk away from the 700R4 and/or the 2004R transmissions- - - - -they're an unreliable PITA! A 4L60 requires an expensive stand-alone computer controller, since it has shift solenoids that are controlled by the vehicle computer on later model applications- - - -1990s and up, IIRC.
A TH 350 will be the simplest way to besome a "shiftless Okie!
LOL!
Jerry
