If you got a couple of real air cleaners, you know, dome aftermarket type [Ramflow?] & ran the tubes to a fitting in the base plates, you should have no real problems in my view.
I guess that you could even run the two hoses to a "T" piece & then to one air cleaner only. That way any hydrocarbons [fumes] will be sucked into the engine & burned. That should do the trick.

I don't know for sure, but i think the A/C on the canister is for Air Cleaner, not air conditioner. The canister needs clean filtered air to do it's job properly.

Thanks Dodgeman

On the earlier round top carbs on my GX engine, these had a pipe that ran down the side of the block to a little bit below the sump gasket level. They are to vent the bowls & provide an overflow for overfilled bowls.

Yours would have been connected to the clean air side of the original SSS air cleaner housing.
Your carbs look like they are not SU's, they are even better, they look to be Hitachi's from something like a 180B SSS [610 SSS] That's how the hoses are shown in my 180B manual.
If thats a factory inlet maniflod, then see if you can find an air filter assembly from a 180B SSS. I guarantee that you will not lose performance compared to the rock seives that you have now.

On 910's [Bluebirds] these hoses were connected to a solenoid system that, when the ignition was on, vented them to the air filter, but when the ignition was off, they were vented direct to manifold vacuum. This placed an equal pressure both above & below the fuel in the bowl. Since fuel only flows as a result of pressure diferential, & this pressure suddenly becomes equal, then the fuel does not flow, & any running-on problems dissapear as there is no fuel being sucked in to burn.

This has the same effect as the idle cut off solenoid in the downdraught carbs.
Very clever those Datsun engineers