1975 Dart 318 (sorry but it's a4 door) Just want to say THANKS in advance the help!!

I know it's been a while since original post but here's an update and another request for suggestions- Got the headers in, carb tuned, and engine is running well! It sounds like a beast without exhaust, LOL! Here's the question, 2 1/4 or 2 1/2 inch tubing for exhaust? I plan on putting x-pipe in and 40 series Flomaster mufflers, thoughts and suggestions?