I got my 73 dart swinger at 18, I just turned 21 now and im still not done! It started out exactly like yours, 318/904/7 1/4 rear end. My 318 was toast, so I pulled it and build a pretty hot 360 (~450hp), 999 trans with a shift kit, and installing the 8 3/4 with 4.10 gears pretty soon.
This...