Foxy, IME, your Speer 70 semi-spitzer may, or may not stabilize in your 12 twist. I shoot 69 gr SMK's out of my 8 twist 22-250, and they shoot WELL ! For whatever reason, my 9 twist Mini-Mauser 223 doesn't even like the Speer 70's, why, I don't know??? I've never even tried the SMK's in the 223, because it LOVES the cheapie Remmy 50 HP's for some reason !! It boils down to - try them and see!
Mark