i would rebuild what you have,4.500 bore makes it a 509,go with edelbrock marine aluminum heads,hp 500 efi hyd roller cam,and 9.5 to 10 to 1 compression,850 holley with single plane intake,and enjoy 575 reliable hp.i have done several like this,you get a lot of bang for the buck.

The paint marks during assembly and torque checks does not necessarily mean they were assembled by Merc, as many engine assembly shops use this method of confirming all bolts are torqued and rechecked prior to finish. It's a good sign of a careful shop procedure and one that indicates a good torque system and check was done on the various bolts and nuts. I have opened Merc Racing engines that did not have this check markings in them and I have opened some that did.

Best Regards,
Ray @ Raylar

Price the Merlin Crate 509's from Teague. They are a nice piece at a good price. They got a bad rap for some problem years ago, but Bob said they are straight up now. 575hp.

There is a local shop that advertises and sells the GM crate 500hp 502. Intake to oilpan with Edlebrock aluminum heads. They advertise these engines as marine replacement engines.
So, do these engines have Inconnel exhaust valves?

The Inconel exhaust valve need and usage is good subject for a whole new thread to explain the "when and where" needs for Inconel exhaust valves!
I think I will get one going here soon as I have some somewhat non-conformingviews of the need for Inconel exhaust valves or such in normally aspirated marine performance uses?? I will see if we can gain some educational and informed clarity on these beliefs?

Best Regards,
Ray @ Raylar

I always felt that, Mercury would not have spent the money for these exhaust vakves in (I think) all of their big-block applications unless they were needed for a long life engine.