Since this is in a 73, it could very well be a vent problem. Read your shop manual, the fuel section on tanks, and the emissions section in the back about the evap system. These changed over the years, so make sure yours "as built" matches what is in the manual
"Very first" thing I would try, is a drive with the gas cap loose or removed
What is the situation with the front 1/4 vent tube, IE is the carbon can stuff still hooked up?
