My 283 has origanal bore,have had two different sets of heads that performed about the same (461'S AND 305'S). Put a decent cam, headers, elec ign., alum intake, holley 600 vac, in my 55 and got 15ish at 94 in the quarter. Runs real good and sounds real good. If you want more than that you may have to go with a bigger motor.