Pressure building in tank

Should my '69 Barracuda have a vented cap and how do I know if it has the correct cap on it now??
Jrc4y4 says that his engine runs fine in cooler weather and only acts up when it's hot so I wouldn't suspect the tank nearly as much as something else...unless it's related to the cap??

treblig
69 has a vent tube on the filler neck that vents thru a metal line thru the rubber trunk floor gasket right beside filler neck. I shouldn't have a vent gas cap