No need for any extra crossmembers, any passenger car 4 speed will be much lighter than the option 4 speed that these trucks came with. The SM420, the granny gear 4 speed, weighs MUCH more than any passenger trans, it used no rear support at all. The real trick is to be sure that the trans is propely aligned in the index bore of the bellhousing and that the mounting bolts are tight. For the shifter, I'd suggest that you take a look at the 4 speed shifter from Patrick's. It's made to properly fit these trucks without butchering the seat. I think that they are made for them by Hurst, but I'm not sure about that.
