I have twin 255's in a 79 Sea Ray 300 Sedan Bridge. I was going to replace the manifolds and risers but reading some posts I had a question. Should I go a head and replace them all or just the risers? I know they can rust through and cause havoc. How often should the riser gaskets be changed as well? I don't know how old they are, it would be good to know I was starting with new, good iron. Any help would be great