if you use a cover for the cowl, does the heater still work? i have two small holes in my cowl (i think) and every time i give it a good washing or a decent rain comes along, i get water in the floorboard.
Heater should work, should work better in fact, constantly on "recirc" without fresh air coming in. Downside is - you don't get fresh air coming in, if you want it.
I think it's also worth mentioning that this is just a band aid, not a cure. Rust is cancer and if you treat cancer with a band aid then it will be out of hand and running rampant before you know it. I know repairing the cowl properly is expensive and labor intensive but take a look from under the dash, if it's the edge of your tophats leaking then you might be able to fix it from inside the car, for now at least.
I'll probably open mine up and replace the metal because I plan on passing the car down.
But I've seriously thought of taking off the fenders and applying POR15 through the drain holes. I'd use a sponge attached to a hanger wire. Then glue in some of those plastic hats to seal it off so they don't leak.
OK, just wanted to make sure you know what you are getting into, the choice is yours how to handle it. I'd hate to see your car brake in half with all the other mods and $$ you have dropped into it. I'm sure you can just imagine how much more down time is involved in trying to weld two halves of a car into one again, especially if the new front clip has to come from a donor car. Not to mention that if it has been leaking for some time then your floors are likely shot too.
It makes it easier to see any damage if you remove the drivers side vent from the top hat and look up at the cowl bottom. Stuffing your head up under the dash with a flashlight and mirror. Also check where the cowl top drain is up under the front fender and at the firewall. Mine was so rusted that a cowl cap and plastic vent "hats" still didn't stop the water.
Heater still works, but volume of air is reduced some when the blower setting is on high. It is trying to suck the air through two small drain holes rather than a wide open cowl. The plastic hats work well if you use plenty of sealer, for awhile anyway.
Unfortunately, when I went over the car 10 years ago, I didn't know these cars had this problem (newbee), and mine didn't start leaking until about 5 years ago. I will fix this properly when I strip it to the shell in about 2 years.