I am using this carb on a 64 valiant. I capped the outlet at the base of the carb (throttle) that comes out under the fuel inlet. My understanding is that this is an outlet that is supposed to go to a gas re-circulation system on the newer models.

My question is; what is the best thing to do with this outlet. It slowly but continually leaks fluid that smells a bit like gas but is more the consistency of light oil.

Should I try to install a later model charcoal cannister and recirculation system? This won't actually work unless I install a return line to the gas tank which I don't want to do. Is there another way to install something that would work without installing the return line? I would love to have something to kill some of the gas fumes that come off of this carb at shutdown.

Please take us a picture to show us which of several hose nipples meeting this general description you capped off.
1973 and later BBDs have a venturi vacuum takeoff nipple for the exhaust gas recirculation system, but it is a very small diameter nipple and it is nowhere near the location you describe.
The only fluid in a carburetor such as yours is gasoline.
The charcoal canister is used on 1972-up models and does not tie in with an exhaust gas recirculation system. The canister stores fuel vapours from the fuel tank and is also plumbed into the carburetor's bowl vent, but the bowl vent nipple is not below the fuel inlet, so that doesn't match your description either.

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