Main things are you move trans mount to th400 location, make sure you have a bracket and throttle arm to match the TV cable, and if you want to be able to manually shift into all detents swap shifters. Also recommended to at least wire in a toggle switch to activate lockup converter
Been there and done that (in the just finished Tudor project). Definitely worth the effort. Great first gear and overdrive, win win. First step in determining if the new set up is a go or no go, is to determine how strong you need to build the trans. Then compile a parts/price list to see if it’s in the budget. Building for +400 hp gets a little pricey. Crossmember, driveshaft, detent cable, and wiring are all standard type stuff. Must admit my fun really began when trying to get the exact shift points I wanted. Trimmed up four different governors to play with. And took a few tranny fluid baths. Isn’t hot roddin fun.
I'm currently not running one but I did when I was still playing with g bodies. And yup mount position is the same as th400. If your current th350 is a short tail your current driveshaft will work fine as is. As for the lockup yup they all have one. I've heard guys say they put over 100k on one without wiring it up but I at least wired it to a toggle on any I did. But again I know an old guy that's been running one in an electric Camino since like 1999 without it wired up and still on same transmission build