what is the difference besides stronger valves? Thanks!!!

On a marine motor you don't have the benefit of fully dry,non water jacketed step tube dry headers which is how most motors are dynoed. You do want good flowing heads with a good intake to exhaust ratio and ports that flow good but are small to have enough velocity which is what will give you good,useable midrange tq. RMbuilder/sb (cfm) know more about it than I ever will in my lifetime so you might want to talk to them. For example-if you have a 502 with restrictive exhaust you wouldn't want 357cc intake runner heads that only flow good at 700 plus lift because they will never reach their full potential and will kill your mid-range tq and throttle response and you also don't want heads that are too small or too restrictive to where they hold you from making as much power as possible. Keep in mind some porting jobs if not done correctly will hurt low lift flow numbers to give you fancy ,usless high lift numbers. You also want the best springs and valves you can afford within reason to avoid valve train failure. Most competent head builders also agree a head set-up for marine use should have a wider exhaust seat margin to help pull heat from the valve and keep valve seal (typically .080 thou) and slighly looser valve guides for long hard runs to keep from valve sticking,Smitty