check for resrited exhuast, and take the distubutor cap off, and see if shaft of the dist move's back and forth. Also point gap good be opening too far. Make sure your timing is at spec. Too much advance loss of power. do compression check, take carb cleaner and spry the manafold, while eng is running. If motor speed raise's you have b lown intake gasket. Make sure the emissions tube is not pluged, causing too much crank case pressure. Good luck let me know what you found thanks ROg!!!

Are you sure the transmission isn't slipping. It should go nearly as well as a v8. The old 2 speed powerglides were a drag strip racers choice back in the day.