[damnyankee36]

I don't think you will find a modern cap with the gasket and a 13lb rating. I would go to a 16lb anyway. It will help keep the coolant in the radiator instead of easily overflowing into the reservoir. It won't hurt the system.

I used to use a 20lb cap in my SBC Chevy for circle track racing. We couldn't use antifreeze so a high pressure cap was the only way to keep it from puking.