Hi Bow Ties,your boat "speeds" for itself and I'm not disputing dyno figures by reputable engine builders.We used to get Guardian Marine crate engines, they were basically marinized engines.Harman and Hardin Marine did the same.They used to put slightly hotter cams to bring the horsepower up from stock.I've built 460 engines with cams similar to your cam's specs and get good power increase with higher compression figures.You must remember the LS6's compression dropped significantly after 1971 when they went open chamber heads.Horspower was still rated at 450HP but you could tell the difference in performance in boats.My friend just sold his 1970 chevelle with the LS6, #match car and it went better than later chevelles.Your speed is in the rigth bracket with your HP figure but I would try to go to an A impeller to get the RPM to get the performance out of your engine.A truly balanced LS6 could take to 7000RPM occasionally.I have the tm2r manifold as well as most LS6's were equipped like that in the seventies.Life was good back then huh?Good boating and dont'loose your hat.