Hi Steven,
I had a similar problem with a boat years ago. I sealed it up and used check valves similar to what you have done. I did not have any leaks afterward but did not have adequate venting. The tank would pressure up pretty good. Curious, how you are venting excess pressure in the tank with this setup?
BTW - thank you for sharing this information. It sure helps others going through these same hurdles building the GT-R. My car was purchased as a partially built project. I was planning a build thread then realized it may be hard to follow because the running gear is already installed, so it would not be a complete build. I may start a few threads on specific areas, ie. GT-R ducktail, GT-R Clam shell, ect.... There certainly are not many GT-R build threads out there yet.