MODULAR BELL HOUSING
vs an adapter plate and conventional flywheel, not much. The Modular bellhousings take all the guesswork out of the proper depth of the shaft. You just center it, bolt it up and rock on.
I put my tranny in back in 2000, before the T-56 "craze" hit, so I was forced to make my own plate, and learned the hard way about centering the trans to the engine, shaft depth, ect. ect.
I lopped off 1/2" off the pilot snout, milled off 3/8" from the front transmission plate, (later found to be unnecessary) had a bronze pilot bearing machined to .750" id and a few other pain in the ass things to get it to work.
Now you can buy the plates, Here's an article.
Chevrolet 305 ci Engine Build - Tech Article - Chevy High Performance Magazine