The Cometic's do better with the seeping on the side of the block from my experience. For salt water, I'd rather run Cometics in your situation but if you don't have the block and heads machined smooth for them then you shouldn't run them. A finish of 50 RA or finer is recommended.

Fel Pro says ........
Gasket Sealing Surface Finish | Technical Bulletins | Fel-Pro Gaskets

Fel-Pro® recommends a finish of 60 to 100 Ra (roughness average) for cast iron cylinder heads and blocks, and 50 to 60 Ra for aluminum. Fel-Pro uses proprietary head gasket coatings and facing materials designed to fill in minor surface imperfections and allow for improved sealing on imperfect surfaces found in the repair environment. OEM MLS gaskets work well on new, flat, clean castings, since they require a very smooth surface finish, usually 20-30 Ra or less. Fel-Pro’s PermaTorque MLS head gaskets use a specialized coating that accommodates finishes as rough as 60 Ra. Surface flatness is equally important to surface finish, so always check components to ensure they are not warped or distorted. To maintain constant contact between the head gasket and mating surfaces, all deck surfaces must start out flat and remain flat after being torqued to specification. A good rule for flatness is that surface flatness, measured in thousandths of an inch, should never exceed the number of cylinders on each bank across the length of the block.

I have ran fel pro composition gaskets, victor reinz solicor, cometic, and fel pro mls in my engines. The 454/420hp engines had the compositions. The 600hp 454s had victor reinz. The 800hp 454s had cometic and felpro mls. The 600hp 540ci had composition. Never had a head gasket failure on any of them.

That being said, Ive never ran any of them past 300 hours. Number of years can be misleading, esp with northern boaters. Some boat owners dont log 300 hours of run time in 10 years.

Run time is more important to me. Head gaskets are subjected to heat cycles, scrubbing between head and block, brinneling, and so on. This is why on many merc engines, the head gaskets start failing at x amount of hours. MLS gaskets are far more durable, with alot more technology in them vs a composition. This is why oem vehicles , from a grocery getter suv, to a high performance factory car get them on the assembly line.

if youre certain youre pistons are at 0 deck, and not out of the hole at all (which i would verify with a deck bridge), than the .039 gasket should be fine. If theres any doubt, Id go with the thicker gasket like Panther suggested. If it was a race car, and you were trying to squeeze those couple extra HP, and dont mind your pistons kissing the heads, you can run less. Being in the .050 range works just fine on a bbc marine engine.

On a running engine, making a good amount of power, especially once you start getting into the HP per ci range, the cylinder pressures get pretty high. If you get even some slight detonation, the cylinder pressures can really high. The cylinder head is constantly trying to lift off the block, and moving side to side as well. The reason MLS are superior gaskets, is they are basically like a spring. The spring recovers from the expansion issues from heat, as well as when the cylinder head lifts ever so slighty. Once you get to that point with a composition gasket, failure is likely , even if it takes some time to get there. A pair of fel pro composition gaskets is like 80 bucks. A pair of MLS fel pros, is like 140 bucks. I personally think that extra 60 dollars , gets you a much better gasket, that in normal operating conditions, will outlive the composition gaskets.

Pros & Cons of MLS and Copper Gaskets

So this looks like a good choice? https://www.ebay.com/itm/2-Fel-Pro-H...taAd4v&vxp=mtr

I get that MLS has advantages but im not gunna go that route this time around.
And yes, my new pistons measure zero deck height. This was the goal.
.039 is a good quench result right?

That's them. Is this just a medium HP build?

I'd rather er on the side of caution and see .045- .050 quench if I were building it but that's just me.

More quench clearance equals higher risk of detonation no? What risk is involved with .039" clearance?