I am just north of Toronto. The machine shop recommended the flat top silvolite piston with the 50 thou head gasket.

Most of those flat top pistons with 4 valve reliefs are about a 5-6 cc "dish" for the valve reliefs. Unless I am doing my math totally wrong with a 1.560 compression height the piston will be about .020 down in the cylinder. That combined with a .050" gasket and the 64cc Vortec heads gives you a compression ratio of just about 9.5 to 1 ? I guess as long as you are ok with 93 octane fuel then it should work.

Machine shop said that would run 87... Stock mercruiser 350 compression is 9.4, but it has 50 quench to help. With this flat top option the quench is out the door.

Many years ago, I had "260 Mercruiser (350 non Vortec GM) in my 23 ft Wellcraft. I removed the engine around 250 hours and installed brand new GM Vortec heads, different valve springs, a mild flat tappet Comp Cam around 212 / 218 @ 0.050. New Edelbrock intake, roller rockers, timing chain, HiPo Mercruiser exhaust manifolds, and kept the stock flat top pistons with 4 valve reliefs and never even removed them. I calculated around 9.3 compression and was concerned as well, since I think I used the stock Mercruiser head gasket. I started at 32* total timing and 89 mid grade. It gained 9 mph and I switched from a 21 Mirage to a 23 Mirage and it turned that prop to 5100 rpm. Later, I put the timing back to 30* total and ran a few tanks of 87 octane and could not detect any detonation or pinging. I went back to 89 for safety and ran the boat like that 8 years, no issues, sold it to a friend and he has had it with no issues for 5 years.

Personally I am a firm believer in the benefits proper quench. The NA and boosted engines I have built with propper chamber/piston matching and clearance have run better and been less detonation prone. Every mercruiser/indmar 350 & 383 has full dish pistons and non-existent quench so they will run fine on pump gas without it, just not ideal.

The last two 350 mags I built were

96 carbed
14096217 'D chamber' 64cc heads
speed pro H815DCP30 12.3cc
balanced factory rods
Zero deck
Felpro marine .039 head gaskets
9.6:1 compression

Pistons aren't an ideal match to the D shaped chamber but have a much wider band around the perimeter to provide some quench. Pistons were all within a gram and ran great on 91 and 89 octane for well over 500 hours mostly pulling with up to 1800lbs of ballast and people in the boat

2004 350 mag fi
​​​​​062 vortec heads - 64cc
KB hyper 12cc Pistons (vortec mirrored dish)
balanced factory rods
Pistons .001 out of the hole .039 gaskets

9.67:1 compression with the heads resurfaced

This one ran even better imo on 91 & 89. Also had 87 mixed in once or twice on the water with no noticeable detonation.

All merc/indmar 350 mags/monsoons will have cam pn 14097395 which is the standard rv/ram jet 196/206 @.50 109 lsa grind.

The 383 hammerhead pulled off 9.7:1 compression with aluminum heads and called for 89 octane. EDIT - looks like they ran a larger 222/230 @.500 112lsa cam to drop the dynamic compression a bit

I would spend the extra money for forged pistons. The 350 EFI's seem to be detonation prone, and many have head gasket failures, leading to burned pistons. Forged may save the engine if this were to happen.