It sounds like the vent line at the tank may not be at the top. Maybe a defective tank or the vent is hooked up to the wrong fitting. A pic of the vent exit from the tank may help.

Also some '70's were "closed vent" with a can in the trunk to separate the vapor and fuel and a line to the engine for the vent. By chance was your's originally set up that way?

With vent installed it should not need a vented cap... and vented caps sold today hold a fair amount of pressure before they "vent" so the cap won't help.