cowl rust problems.

Thats a sucky problem you have there.

As stated above you can close it off but you wont have any fresh air ducting and your heater wont work well. Any chance to locate good used pieces and install them ? Sounds like work i know - but it might be worth considering if your car is valuable enough. Remember that these pieces will work from any style 66 A body valiant, 4 door dart, etc. These pieces are probably spot welded in, but Im not 100% sure how it all goes together.

74 sport is 100% correct about keeping crap out of that area as well. make sure to flush water thru there and see that it drains out properly.