I appreciate you taking trouble to read my thread and offer advice.
The TH400 is one of the few good things about the car, no way would I part with it [its rebuilt from a '79 Pontiac]... Oh, wait you're talking about selling the trans and engine from an actual 442 coupe! Sorry, good advice.
I might consider changing engines, though. The 403 has a lotta good top-end, but is a bit lacking in torque for a 4300lb vehicle. It has only 25K on it since last rebuild [but they didn't do the valve guides; int'l vacuum leak]. A 455 would be nice, should fit nicely.
I bought the rear a few years back [it used to have a diesel-geared rear] -- although if a posi or limited-slip became available, a switch may be conceivable.
The body is not in the greatest of shape, so finding a good 'donor' body might be desirable. I wouldn't want to do anything that would change the VIN, though because she was originally a diesel, so she's emissions-exempt in VA -- I can do ANYTHING to the engine that space and funding allows.
Don't know how difficult switching from a steering column-mounted auto shift lever to a floor mount would be. Gonna need help on that one.