Well, I don't have to worry about it for now as it turned out to be totally gas tight, I used a more sensitive pressure gauge for that test as Van's was warning about testing the tank with pressures beyond 1 psi. I tested it with 8 inches of water, which is about a 1/3 psi. It actually later turned out that the tank was so gas tight that the pressure rose up to 12" as I had filled it with cool air from the shop A/C and the system warmed up over night and therefore increased the pressure.
Initially the tank leaked somewhat but it turned out to be leaking around the fuel neck flange and the Moeller gauge hole which I had covered up with a plate. After tightening the screws on these flanges the tank did not lose any air in 12 hours.
I hooked up the pressure gauge to the fuel outlet of the tank and plugged the return line inlet. The rest as per plans in addition to covering the hole for the Moeller gauge.
