My plans, your thoughts
I know you said you don't want to cut the Roadrunner/Super Bee, but get yourself a copy of the current issue of Mopar Action magazine. They've got a feature article of a B-body Roadrunner mounted to a current Charger floorpan. The major changes involved shortening the Charger wheelbase by 4 inches to match the Roadrunner and adding sheetmetal to the trunkfloor area. Sure, there were a bunch of other changes but the entire Charger interior was fit to the Roadrunner.
