please tell me you aren't using gas that is that old.

does backing the timing off help?

running premium gas?

seafoam is going to lower the octane rating a fair amount. so if it is pre detonation, that might not be such a great idea.
I've had healthy engines diesel themselves all over the place after a heavy seafoam treatment.
just because it sat doesn't mean there is carbon build up. if anything, your compression would be low because the cylinder walls and rings rusted.
Take the spark plugs out and look down the tubes, you should be able to see if it is really nasty in there. if it is, pour a bunch of seafoam or carb cleaner or whatever down there, break it all up, and crank the engine over (with the plugs still out, and after letting it sit for a little bit) let it push all the excess out (might make a mess, be prepared) then put it back together and start it.
another thing, pull the fuel rail off and check the spray pattern. an injector that is dripping, or barely spraying, or even just constantly spraying just a little bit, will cause detonation issues. a bad injector is a possibility with an old engine. it could still ohm test within spec because the injector guts are good, but there could still be corrosion problems.