Is the 383 a better engine, or does the 440 just have more displacement?

Same head, same HP potential with the power higher (383) or lower (440) in the powerband. Same as the slant. Same head, 3 displacements. RB is ~2" wider. Id build a high compression 400 or even a 451 over either: less valve shrouding and better rod to stroke. 451's are cheap now the crank are available off the shelf.