On my 68' I went from a sm blk. p-glide to bbc th400 to finally a bbc Muncie. When I added the short shaft 400 I fabbed the second set of trans. frame brackets which were place 6.5" to the rear of the stock brackets.
Some model's -I don't know which, I think the factory th400- came with 2 sets of trans. mount brackets. I can get pics. if you need them.
The TH400 was introduced in 1965 behind the BBC 396. Every car made that had a big block as a motor option should already have the holes punched in the frame already there to mount a TH400 or the earlier PowerGlide (or manual transmissions) which all shared the other holes punched in the frame. You just slide your existing transmission cross-member back three inches and the correct size TH400 should bolt right up. I say correct size because the TH400 comes in three different over all lengths with a 3, 6, or a 9 inch tail shaft to fit into small, or large cars and any of the light trucks.
There is a physical difference in the construction of a TH400 cross-member compared to the TH350 version. The TH400 cross-member was made out of a bent round tube with a bracket welded in the middle to mount the tranny and two brackets on each end to bolt it onto the car's frame. The TH350 cross-member was made out of two pieces of stamped steel that were spot welded together. You can use a TH350 cross-member on a TH400 transmission but you will need to grind the front lip smooth to get the oil pan off the transmission without taking parts off the car as it will interfere otherwise.