Is the one from Ames correct for '64? From what I've seen, it fits the bill for a "correct" '64 radiator cap. At $26, it doesn't put a huge dent in your build cost.
I've seen "original" AC caps on ebay for hundreds of dollars. I'd challenge anyone to discern one of them from the Ames version, P155A. The most obvious difference would be that if the "original' cap was truly NOS, the zinc plating would be faded from age, whereas the Ames cap would be bright zinc like when the caps were new in '64.
Here is one of them on eBay right now.
https://www.ebay.com/itm/NOS-Origina...hUo~Aa&vxp=mtr