Marc, actually the 511" FE I used in Drag Week, and shown in the dyno results section, used 6.8" rods. The only downside I can see is that sometimes with a 6.8" rod the pin will protrude into the oil ring groove, necessitating the use of lower rails in the ring groove, to support the ring. With a 6.7" rod and 4.25" stroke, you don't have that issue, so I think that is why it is the standard combination.