that problem is encountered alot on the ford v/8, usually caused bu poor connections and grounds, make sure you have a battery with enough cranking amps, that you have 6v cables the thick ones, at least #4 cables, not the thin 12v cables, make sure all connections are clean and tight, run the ground cable to or as close to the starter as you can, then run a cable to the frame, then one back to the engine, while your at it make sure the body has a good clean ground, do all of that and your problem should be fixed, unless your starter is weak, excessive heat on the starter can make it turn slow
