Generally speaking, mileage is not an indicator of remaining useful life in a transmission.

Operating cycles at high power levels are.

A drag car puts on a 1/4 mile at a time and cycles the parts causing more loading than driving a car 100’s of thousands of miles.

And in high gear the geartrain sees no wear and tear.
Just bushings in the pump and front clutch drum, rotors, and the output shaft bearing.

Again, a seasoned C Body trans would be my first choice for a core.

All 727 trans main cases are the same dimensionally, none are better than others. The only thing you may observe on cases is that some have additional external webbing and that is usually for truck applications for off-road and 4 x 4 use.
Not for torsional issues but for vibration and cracking due to severe jarring.