There is no OVP on a 75 450. Check the distributor and see if you're getting any advance when you rev the engine. I had a 4.5 with similar syptoms caused by a frozen distributor years ago. Not only was it slow, but the exhaust manifolds would "glow" bright orange. You can also put a vacuum gauge on it to check for a plugged exhaust.
Regards,
Randy D.