While the 426 Hemi's were based on the RB motors there are extra headbolts that screw into bosses located inside the lifter valley. At one point several years it seems like there was a company that sold a kit to convert the RB blocks (basically a plate that screwed into the lifter valley that provided the necessary bosses for the headbolts) but it seems like this still resulted in poor sealing and blown head gasgets.

Good luck in your search Bob, I wish I had a lead or 2 that I could give you (but if I did I'd likely buy it for myself )