I had leaky head gaskets on my old 4.3 V6 but the way they failed caused water to leak into a cyl, but luckily for me did not allow exhaust gas to scorch the block deck surface. So I was able to do a top end overhaul with the engine in the boat, after meticulously cleaning up the block deck sealing surfaces and cleaning out all 26 of the cyl head bolt holes with a thread chaser and solvent and compressed air. I installed a set of reman 4.3 V6 heads, Fel Pro marine head gaskets and used better quality ARP cyl head bolts. I did this repair in 2017 and it's still running well. The short block is all original.
I would advise new if you can get it, you don't have to go through Merc, there are less expensive sources for brand new GM marine 5.7s. Also, I know I'm spending your money but I think you'll have longer engine life with at least a half closed cooling system. When I repower this boat I am definitely doing that and here's my reasoning.
Raw water cooling for an inboard is at best a cost saving compromise.
These engines were designed by GM to be in an automotive application with a pressurized cooling system and antifreeze with corrosion inhibitors. An open or raw water cooled system does not control temperatures throughout the system as well as a pressurized system does, there are hot spots especially with the cyl head design of the GM/Chevrolet small block. You can see that the center cyls exhaust ports are right next to each other, this causes this area to run hotter than the other parts of the cyl head. The V6 is a bit better in this regard because the exhaust ports are more spaced out. In the second pic below you can see that the exhaust valves are not right next to each other like on the V8 cyl head.
Even in fresh water I feel that closed cooling is superior, better control of engine temps, engine runs at the temp it was designed to run at and they are also a lot easier to winterize. No pulling apart the rear of the boat to get at those drain plugs . You can just suck antifreeze up the outdrive with this system with no risk of raw water still being in the block like raw water cooling. Or simply drain the head exchanger, manifolds and raw water intake hose/impeller.
Outboards were designed to be raw water cooled, inboards were not, IMHO...
Here's a few pix of my little project:
https://www.dropbox.com/s/2pu8ac8x71e5u ... l.JPG?dl=0https://www.dropbox.com/s/8fhnjbtutfjej ... 2.JPG?dl=0https://www.dropbox.com/s/512kk3jkk1klb ... d.jpg?dl=0
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