Putting my 1960 Rambler American station wagon together I had to (had the luxury of) choose between a recently rebuilt M8, the cast-iron boat-anchor that is correct for the year, or a 1962 M35, light duty aluminum cased transmission of unknown but decent-seeming condition ("ran when parked") that was in the car when I got it. Fluid is clean, the torque converter is very clean, and the car had been cared for by a responsible owner.
It came down to the supportability of the vacuum modulators used in the iron transmission. They contain a rubber and fabric diaphragm that eventually fails (decades of use). Alas, new ones are not available, rebuilds seems scarce or mythical, and replacements seem to be more used parts.
Mantra 7a: Old rubber is bad rubber. Universally, NOS, NORS, old is old.
So I took one apart to see what the story is. Photos below. My assessment is: this is not a part that I can rely on in a daily driver. If it was a simple flat diaphragm, maybe, it would be more esaily rebuildable. But it's a formed cylindrical diaphram. There were rebuild kits for these, but even those are old (see mantra 7a above).
The M35 weighs close to 100 lbs less. The engine has 130 hp, so the additional ruggedness of the iron box is moot. It's too damn heavy for the car. It's huge. The 1962 M35 is plug-compatible with the M8; same overall length, same output shaft, same bell, same mounts, same selector lever, same speedo cable. Requires adaptation of the throttle cable, 1962 parts. Last, the transmission shop says the M35 is a more supportable choice. Done and done. YMMV.