Go buy a new 60s Ford pickup gas cap .
Drill out the center allowing the rubber seal to come free.
It will fit over your stock cap and tuck down under the stock center.
I did this on my 67 and it has sealed very smell for 7 years
Like this one
NEW gas fuel cap 1949-59 Ford car, 1951-1971 Ford pickup 8A-9030 | eBay