i've learned ANY holes in the firewall or tunnel cover will wick engine fumes up as well - plug tight any firewall holes.
plus if your carb is running lean / exhaust leaks it makes even worse
replace the gasket on the tank sending unit
install a new vented gas cap - i am not certain on these early ones, but assume vented
plus if your carb is running lean / exhaust leaks it makes even worse
replace the gasket on the tank sending unit
install a new vented gas cap - i am not certain on these early ones, but assume vented