I also just bookmarked this thread: https://board.moparts.org/ubbthreads/ubbthreads.php/topics/140863/all/727-to-a518-swap.html
and will also be calling http://www.phoenixtrans.com/

I like the idea of the shop "just" welding a RB bell onto the 518 and eliminate the spacer.
Our 73 Jeepster had, all original, an AMC 304 and TH400 trans with OEM adapter, which cracked 2 flex plates due to imperfect machining/alignment. Argg.