the most IDEAL engine would be the longest rod combined with a stroke under 3.5" (providing the piston pin dosn't go to far into the rings). you want rod ratio's in the 1.8-2.0:1 range. combine that with a bore larger then 4inches. those types of engines breath deep, and rev high.. the ticket you need for high horsepower and flat torque curves on a naturally asperated engine.
cheers