I'm still working on improving my gas tank vent system. I found a couple of sources of decent info on how a modern evap system works.

EVAP Evaporative Emission Control System

https://www.youtube.com/watch?v=AlfOSMmisPg

I don't really want all the complication of an evap system, but I do want to keep the stink out of the car.

I've got a high-mounted marine flip top gas cap that isn't vacuum tight like a modern auto gas cap, but it's only vapor leak source is the small clearance area between a large spring loaded washer (with rubber sealing gasket) and the 5/16" shaft that it slides up and down on. I'm sure that the area is no more than that of a 1/16" diameter hole and not enough to vent the tank under normal operating conditions.

I ordered a remote mount roller-over vent valve that will be mounted about as high as the filler cap, but even with a vent hose routed outside the car, it will still stink up the garage, with the 1/4" hose open all the time.

I got a 1/4" solenoid purge valve with the LS motor that I bought, so I'm thinking of using it as a vent valve, to allow air into the tank only when the engine is running. Since the gas cap is not totally sealed, I won't have a problem with temperature changes causing the tank to swell or contract.