I've installed a beefed 2001 Viper T56 in my 71 Challenger. I got mine from Geoff Gerko at Gerko Enterprises in Lancaster, California. He has made me a custom aluminum transmission crossmember that bolts to the factory 4 bolt holes,has excellent ground clearance, and allows you to use 3 inch pipes. He'll make them out of steel as well.
We're using my car to design the upper brace. (The part of the steel frame you have to cut to get the T56 in.) It allows you to still use all 4 factory bolt holes. Once this is finished, this part should fit all E-bodies, and you won't have to fabricate this part yourself. I think he will manufacture slightly different ones for those who run a slim version of the reverse lockout solenoid, or no RLS at all. It should be finished this month sometime.
Joe