Is it possible that a factory SS conv could come equipped with the above drivetrain?
And what is (are) the major differences between the TH350 and the TH400?
Any help / insight would be appreciated...

I think the 400 turbo was mandatory with the automatic 454. The 400 turbo is a heavier duty transmission. The 350 turbo will allow more horse power to get to the rear wheels though. If you build that 350 turbo up right it will hold up for a while. The transmission
mounts are different also. I'm sure somebodyelse will jump with anything I missed.

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Malibu 307 (aka 406 smblk)
69 SS 396(aka 454)
Wolf in sheeps skin

from what I understand, the physical gears in a TH400 a huge compared to those in a TH350, and since there's more rotating mass, it takes more power to turn 'em. the gears in a th350 supposedly take 15hp to turn, while the gears in a th400 take around 40hp. the two trannies weigh almost the same - 130lb for the 350, 135lb for the 400. there's a tranny shop by my house that'll put the 350 gears in a 400 while retaining the 400 case, valve body, etc. it costs $400. But you get a strong tranny with more real wheel power. personally, i myself am going the th350 behind a big block route.

The TH400 was the only automatic tranny that was mated to the big blocks 454's. Anything else is not original!