The longer the rod the less side-loading it does to the cylinder wall...I wouldn't do an RB with anything less than 7.1 rods IMO.
Yes, less side wall loading but what I really like about the longer rod is that it keeps the pistons out of the crankcase. The further you can keep the pistons away from the big rotating cloud of windage the better. Some engines such as SBF and BB Chevy engines make do with short rods. The short rod motors run hard, but given the opportunity I prefer to use a long rod.
