Chris:
It's a '73 Dart Swinger 5.7 with a 76 mm turbo. Powerglide with 8 3/4 Mopar diff. I use a MegaSquirt ver. 3x. The set-up and tuning has been difficult, but hopefully will pay off. The Megasquirt worked great with my old motor, and for thousands of other folks too (
www.msefi.com) I assume you mean the factory trans and diff behind your Magnum. If so, the trans can be beefed up considerably. The diff., however has its limits. the heavy hitters swap in a 8.8 ford center section ($$$). Check out
www.lxforums.com for more info.
Dave