Its pretty simple, the more cubes, the more tractable 550 hp will be. A 535 or 541 cube motor will have that off idle torque you want, and can use a much tighter converter and tamer cam. I would not waste a second deciding to go big stroker after having built 408, 446, 475, 499, and 528 motors. My biggest mistake was selling a 4.5 crank that could have made a 572 out of my current motor.