Finished installing the new transmission today, took a while as I had to do loads of other small jobs at the same time including fitting a new transmission cooler and resolving a shifter issue due to a bent cable end.
I moved the stock crossmember back 70mm, using the original rear holes in the sub frame as the new front holes and drilling new rear holes. To position those I stuck a long bolt through the original rear hole so I could measure the centres properly. Worked out well.
I was lucky, in that my car originally had a TH350C with a lock up converter, so I already had the brake switch and +12v wire ready to hook up to the trans, just had to solder on the correct connector.
The UJ is going to be very close to the reinforcement in the floor pan, it should have around 1/4" clearance, I could gain some more by elongating the holes in the rubber mount if needed. Reason is that the engine and trans sit offset by about 3/4" in the second gen, so it's always going to be rather close!
Still waiting for my driveshaft so I can actually test it properly (due weds), but I've filled it with fluid and run pressure tests in park, getting 110psi with the TV hooked up, and it spikes up to 200psi as soon as the cable is pulled. So far so good - and yes it sounds great on open headers