When I did my 508 RB I went with the Manley 6.76" rods mainly because the only 7.100 rods out there were the Eagles and the big ends of those rods were Narrow (around 1.007" vs 1.012") When you offset grind a crank the rod journal winds up being slightly wider by the time you finish machine it so with a wider journal and narrower rods the 'tolerence stack' was going the wrong way.

I think there's more 7.100 rod 2.200 journal rod options out there than there was almost 10 yrs ago so I would probably/maybe opt to go 7.100 for an RB. But that 508 with Ported Stage V's is a BEAST and it's still running strong in Paul's (PHJ426) 72 Road Runner.

The 4.25" in a low deck works amazingly with "only" a 6.535" rod, so I wouldn't stress too much about rod length either way.

Cap walk is of course amplified by detonation and it has been proven that a shorter rod is generally less prone to detonation, get the combination right and tune tune tune to optimize longevity.