I saw two old Stingrays online, one with a 350, one with a 454, and they both had 350 turbo transmissions. I don’t understand how a 454 and a 350 could have the same transmissions. is where the transmission mounts to the engine the same size on both engines or something?

yes they are,they made both versions of the 350 turbo and they made them for big blocks also, any small block chevy transmission will bolt right up to any small block engine,they been that way actually since 1955,and didn’t change until recently,some came with 350 turbo,s in them, some came with 400 turbo,s and and the 700r-4 will fit it also,and so will the two speed power glide from 1955,there’s nothing wrong with a 350 Turbo if its built right, they are a good transmission,most 454,s i have seen had the 400,s in them but a lot was built with the 350 turbo,it was a very good transmission and remains to be one of the best you can get to this day,good luck.