STAB on Ghost-type moves wouldn't have been that big a deal in RBY. The only move it would affect is Lick.
If I could have chosen a pokémon to wind up part Ghost-type in RBY, it would probably be a Normal-type, since Normal's only weakness is Fighting and Ghosts are immune to Fighting. Ghost's only weakness back then was Ghost, and Normal-types are immune to Ghost attacks...so, pretty much a weakness-free pokémon who gets STAB on moves like Body Slam, Tri Attack and Hyper Beam and is immune to Normal, Fighting, and Lick.