Only one flex plate available for 727/904, ATI. 904 can be built to handle big power, equals expensive. 727 is heavier and takes more power, but, spend virtually nothing to build(not being stupid horsepower). Looking at doing different cross members and driveshafts. I admire your tenacity, but, you might get sick of it.
There are several flex plates available, based on the engine used. Mopar has many different engine balances for both big block, small block, and probably the gen 3 Hemi