If you're trying to vent the tank to atmosphere and it doesn't have a vent, you could always drill a small hole in the cap. Caps are relatively cheap. Might be more reliable than a loose cap.
I'm using a $13 summit overflow can (plastic) mounted behind the passengers headlight. It comes with a top fitting, bottom petcock, and is ventilated to atmosphere. The only mod was to pull off the top fitting and add a siphon tube to the bottom of the tank. Heater hose runs to my filler neck, which is a gessford filler/thermostat housing that places the fill point very high. I also do not run an expansion tank. This has proven to be a trouble free solution.
But, it certainly doesn't give you the appearance of vintage-correctness by any means. I'm not into that. My powertrain is the sort of thing that gives true vintage-correct restoration people nightmares. LOL
