If 12 volts won't crank it, it's definitely time for a rebuilt starter! It sounds like a shorted armature. BTW, the "Super-Torque" starters we used to use on the high-compression race engines were just standard Delco 12V starters with a 6V armature in them. Then somebody got the bright idea to adapt gear-reduction Jap starters to Chevy engines, at about half the weight!
Jerry
