74 Dart 318 to 383 Engine Swap Help

Instead of a 360, just save your coin and put your 383 in it, even better. No problem with putting a mild cam, 4 barrel, and dual exhaust in an A body. It will still be a nice running car, even with stock converter and 2.76 gears. How can I say this in the face of all that math? I have done it to my son's 73 4 door Dart. He loves it. 20 mpg at 70 mph all day long. Much better than the 2 barrel and small 318 cam.