....The Cowl Drain Tube comes down from the cowl into the engine compartment and then takes a 90° turn to go out the kick panel, which is where the extension I came up with connects....

Upon re-reading my post, I noticed I wrote ENGINE, and that is incorrect, change it to INTERIOR.

Oops

E

Would it be possibleto direct the effluent through the firewall instead of the side?

I was wondering the same thing tomohawk, With recent rain, water was coming into my car uninterrupted. I looked and I don't even have a drain tube on my drivers side. I was wondering if I should just do a tube out of the front and maybe plug the original, or if taking the fenders/dash out and redoing the tubes a better idea. I wouldnt mind the tube in front and the connecting wall of the firewall doesn't seem to be any higher or lower than the drain holes. Not sure yet

There was another thread on the same idea, where someone used a 1 1/2 dia corrugated drain hose instead of the PVC elbow, and another where the person used an elbow of radiator hose to replace the missing rubber one.

Just for what its worth . I have seen several early Zs that had a hole rusted through the base of the air inlet on the r/s cowl near the r/s wiper transmission. You may not be able to see this without removing the cowl vent grill and drilling out the spot welds that hold the deflector into the upper cowl panel above the air inlet. The original factory seam sealer dries out and splits and the moisture sits there and just slowly eats away at the flange.

I used a little mirror to look behind that deflector. Doesn't everyone have a micro- camera on a flexible stalk nowadays?