My 355's are using the stock Vortec Heads relieved for the Comp Dual Springs. I used the Comp 270 roller and the Crusader/Volvo dual pattern (built by Edelbrock) with an Air Gap, Dyno runs in ideal conditions with a Holley 750 DP and street automotive headers you can get HP in the low 400 range and TQ about 450. Reduce that HP number by 6~10% installed with stock marine exhaust and a BBC Q-Jet. As you know...the magic mover in a boat is TQ as opposed to HP and the stock Vortec heads are a lot of bang for the buck.

If the block is old and rusted don't spend the money boring it and buying pistons. Buy a L31 '98 and later Truck Vortec Long Block...save big bucks in the long run. If the block is decent then go for it.

I had the same thing happen as the OP. I rebuilt a 355. I used a comp XM 262 flat cam with vortecs and an air gap manifold. What should total timing be with vortecs?

No more than 32* total...in fact 30* is safer...that's what I run and I have no pre ignition or detonation on 90 octane RV Gas. Mine is locked in at 22* with my DUI Distributor and I set my initial at 8*.

The Vortecs are very efficient like a "Fast Burn" head and burn it all at 30~32*. Any more and you will eat valves and pistons.

I am at 28. Is there any downside? I am running 93 octane.