Overheating when driving and gas tank building up pressure

[Adam1966C10]

Almost done with my 66. I've taken it on a few test drives and it starts to overheat when on the highway (got up to 230) and it isn't even hot here in Texas yet. It will sit at idle and the electric fan will cycle just fine. I have a newly rebuilt 350, new aluminum 2 row radiator with a single fan mounted on an aluminum fan shroud from Brothers Trucks. I have the Fitech efi with the fuel command center. The timing has been double checked so it is not that.

The other thing that is happening I'd the under bed fuel tank some how becomes pressurized. It starts to bow out on the top and bottom. I have a tanks Inc vent on it and can see capers coming out of it, but it can't keep up. I have to remove the gas cap to release the pressure. The fuel command center has a return line. I pulled it and is doesn't seem to have enough pressure to pump up the tank.

Any advice would be much appreciated.