I like where your project is going. If I were doing this project, I would use the Revell '70 Road runner front bench seat, and get some .030 half-round styrene strips for the pleats, and some .080 sheet styrene for the flat surfaces. I would also use the flat styrene, glued on to reshape the head restraints on the Road Runner Bench. The piping can be replicated with sewing thread. I used those techniques when I did my '67 Chevy Concours Estate Wagon a few months ago.
As far as the engine goes, the AMT '71 Duster 340 kit has a very nice small block engine with the A/C accessories, even though it has a 4-speed transmission. Either the Lindberg Dodge 330, or Plymouth Belvedere (Stock version) has a Torqueflite 904 transmission.
One of my future projects is going to be a '72 Charger Rallye 440, to bookend with my '71 R/T 440 and my Hemi R/T. So many Chargers, so little time!!!!