Where does the vent tube vent to? Does it go up into the roof? because a vent tube has to be higher than the tank in order to work properly.
I know the 70-71 tanks have a expansion type chamber in the roof pillar with a hose coming off that, that vents outside the cab.
I never checked one out thoroughly, I'm assuming it's similar to a pollution charcoal filter in design.
I still don't see why a vented cap wouldn't work on yours, shouldn't be any different than the later ones. If I'm understanding you correctly, a vent tube in the filler neck has nothing to do with venting the tank when the cap is on the filler neck, a vent tube on the filler neck is for venting the tank during filling only, the same as the newer vehicles.
That is one of the reasons filling a sweptline is a bitch, because they don't have the vent tube on the filler neck and it is impossible to fill a sweptline with the pump nozzle fully open.
Vented gas caps are spring loaded and fuel slosh should not open it, even if fuel got into the fill vent, which it shouldn't, the fill vent has to be higher than the fill opening of the tank or it wouldn't work, which goes back to the fill vent only being a fill vent not a vent for just expansion or vacuum.

Mark D.

1968 W200 Sweptline
1969 D100 Utiline
1971 D100 SE Sweptline
1993 D250 Club Cab Sweptline
1986 W250 Cab & Chassis
1978 D300 Sweptline

E. Brookfield, Massachusetts