Scarlet the 4-eyed 'cuda

I have a 1971 Plymouth Cuda. Stock is a 383 HP big block, 727 transmission, 3.23 Sure-Grip diff (489 case). It is a 2-fender tag car (highly optioned). Highlights are A/C, power brakes, front disc brakes, rear defroster, and AM/FM radio. I bought the car complete and drivable.

I am doing minor restoration work as well as some performance and efficiency upgrades. What does this mean? I love driving the car and it appears to be a numbers-matching car so I do not want to ruin it. Therefore, the 727 transmission and 383 block will be removed and set aside. In their place will be a 47RH transmission and a 400 CI Big Block. The 47RH with Ultra-Bell and upgrades along with the 400 BB with either a 451 or a 470 stroker kit will go in their place. I don't know if I'll get there but hopping that when done I might be able to hit 20mpg on the highway (3.23 gears, OD and lockup and a well tuned carb are going to be key).

Thanks, John.