If I'm building an engine for power then I always use the longest rod that will fit. I know that a lot of experts say that rod length doesn't matter but I prefer to use the longest one that works. The EMC engines went the other way on this starting a few years back but they operate under totally different rules so it makes sense for them. In either case I don't think rod length is the issue with your engine. A 511 low deck with 6.535 rods doesn't have the greatest R/S ratio in the world, but it still shouldn't have any issue hammering out 700+ hp.