Buddy when I talked to Spud about SBCs and Nitro---one of his intresting observations was that he learned the hard way not to mix block and head materials with Nitro---alum heads with alum block cast heads with cast block other wise he could not keep head gaskets in them
One of the problems with dissimilar materials is the difference in thermal expansion rate. For instance, an alloy head with a copper head gasket piled on top of an iron block, creates a scenario where all three parts expand & contract at differing rates. This "creep rate" differential can, if large enough,
shear the gasket attachment to the head/block,
regardless of the clamp load. This can be a serious problem, hence the tendency for the "racer's adage": iron heads with iron blocks, and aluminum heads with aluminum blocks. It removes one differential of thermal expansion from the equation.
If you are pushing the copper gasket out, even with O-rings & receiver grooves, then either the combustion pressure is too high, OR, the clamp load is not enough,
for your application.Kinda like: "Which eye do you want poked out?"
I realize you are where you are, and may not be able to change much in short order. Did you ever assess the clamp load via the carbon paper/pressure sensitive film method? Doing so now may point you in the direction of a quick fix, or at least help you identify the where the real issue is.
Hope this helps.
Fordboy