I believe all three of your sixes of choice are seven main bearing engines. The Ford 300 has the shortest stroke/largest bore, and can stand more RPM than the others. The slant 6 is short stroke, too, but not much for cubes. The 230 Chev has been known to rev, it's a short stroke job. My choice would be the Ford first, then the SS, with the GMC last,even though it's a torquer.