I replaced my original tank with an exact replacement. There is a tube on the passenger side top that seems to me to be a vent but it attaches to the 5/16 return line from  the fuel pump.  The cap is also and exact replacement and non-vented.  The tank has no way to vent in my opinion and when I take the cap off it hisses to relieve pressure.  Could this also act like a siphon and empty the carburetor bowl?

If your car has a charcoal cannister it will need a vent line to the cannister.  If not, then the vented cap is the way to go.  You could actually use that either way.

Thanks,

I switched to a vented cap that also has a relief button on it. Worked out great!

No cannister,  yep, use the vented cap.

I ordered one from a 69 that is vented...............

I don't think the vapor canisters were used until '71, so, theoretically, your '70 should have a non-sealed, vented cap. However...everything I can find shows the '70s to have sealed caps....

Your fuel tank doesn't have a fitting on the driver side top??

No, my tank does not have a fitting on the driver's side, only the passenger side which the 5/16 return line attaches to. I still have my original tank, and it's the same way.

Perhaps the '70 models were mix-match. The Gov. insisted on not having fuel vapors vented into the air, so they forced GM to install the sealed caps, before they had researched it thoroughly. Since the '71 year had the introduction of the vapor return canister, and the lines to the tank, we can assume that the large majority of the '70 models, since it was a short production year, didn't get the new/improved equipment.
Just thinkin out loud...

Makes sense................................I think the new cap will cure my problem.