TH350 and TH400 are the same thing/same fit...there is no difference.
Simply put you'd need to buy Buick/Olds/Pontiac bellhousing versions of either tranny if you're bolting it to a Pontiac block, and you buy Chevy version of either tranny if you're bolting it to a Chevy block. OR...you us an adapter plate to do the conversion if you're trying to mix/match a BOP engine/trans with a Chevy engine/trans.
Exactly my point, but why spend the money on something to adapt a 350 tranny to a pontiac, when you can buy a pontiac 400 that bolts to a pontiac without a conversion and have a much better and stronger tranny for little price!!!
