The aluminum case OD do have the floating cluster shaft and wear easier. Cast Iron case do not have a floating shaft. First gear is a very low gear at 3.09, hence no need to even use it with a big block. The only real difference if put in a cast case, is the main shaft is thinner for at least one of the gears. So if anything was going to break it would that in my opinion. This is not a setup for track car, but inexpensive option for a cruise OD car.

I have a 383 4 spd B body that has a dana with 3.54. The motor trans are stock set up. I hardly use first gear even with a stock 383/4 spd.

With the over drive, I would expect it to be two speed with overdrive. But if you wanted to pull a cool hole shot in first gear, what a ratio with a 3.54.....