if i remove the vent assembly from under the dash; the one that has the little doors you can open up. if i weld a plate over the hole, will i eventually have an issue with water pooling up in there or anything? how do the drains on the cowl work?

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70 Plymouth Valiant 4dr

As I said before, is better to close the entire cowl vent with a pieze of metal, heater system will still work and if you open the side doors air flow will increase when used the blower.

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Mopar Muscle is well defend by Slant Six!

Valiant Duster 1971

If you weld the plate to the bottom of the hole you might have pooling issues. There is no convenient way to weld to the top of the hole. The hole has an upright lip approx 3/4" that prevent water from running into the interior so it you weld to the bottom of the hole you will have a cylinder 3/4" deep and 5 or so inches in diameter. What do you want to remove it for? In the winter I just shove a sweater in mine so that air can't flow through as easily. Some others duct tape theirs to prevent drafts.