It should hold up perfectly with the head and block skimmed, but so would a composite gasket at that power level, and have a vastly better chance of sealing, with any imperfections, of course there shouldn't be any.
It can come from other places though apart from the HG allowing the oil water to mix, such as the original fit oil cooler modine, is he still using that ?, I would guess not but you never know.
tabetha