Used to have a 73 similar to yours. Best of my knowledge there is no 4 speed that will bolt in and hook up to that transfer case. Might be able to do something with a transfer case swap.
I know of a BWT10 swapped in by a guy with plenty of machining, but it certainly was not an easy swap. I ran the 3 speed in mine with a floor shift conversion. 3 on the tree was pretty sloppy. Hot setup was the C4 automatic. We were running high speed trails, but if you are planning on rock crawling, there really is nothing wrong with the 3 speed manual. It's pretty strong.
