I'd leak the car down and see if its leaking past the valves or something.
1-5/8 headers are fine. Or at least I'm running 1-5/8 Hookers on mine and they work pretty good.
60' time is very poor as you know. If its not spinning the tires, its probably in the carb or the converter someplace.
Try running it without the power steering belt, and that clutch fan is also a serious drag on power (like a tenth or so).
When I converted my car to manual steering it saved about 50 lbs off the front end!
Steve