Personally, unless you are looking at racing, there is not a much better substitute for the (I'm daring an aSSumption here) marine version HEI I'm figuring you have. There is a high-RPM few-degree retard I have heard about with the system but does that bother your aplication? I THINK the retard starts to happen above 4k. I'm still running single points over 70mph, near 5500RPM.
I've been running a 454 jet drive boat for 27 years now (actually owned dad's boat now 2 years) and have had ZERO maintenance/performance issues to warn you about. Gear your performance mods to your needs, mostly torque at moderate RPM to have good acceleration. My exhaust outlets are 3.5", likely to become 4" with next winter's upgrade to Through-transom headers (Lightning) 3.5 to 4" is good for you.