Continuing thinking along tese lines, I bought a combo fill vent for
my sailboat. I called the manufacturer and asked if I could purchase
the fitting I needed to pressure test the fuel system after I get it
installed. They sent me the fitting right away, for free.
I have the Attwood 3682-1 in stainless. If you can, you should test
your fuel system at 3 psi and check it using soap bubbles like the
regulation says before you put gas in the tank. It can't hurt if you
do it correctly.