428coupe

Well, I am not familiar with the aftermarket a/c units, so I dont know where they get their air intake from. Are you ever going to be running your heater? It draws it's air from the cowl area. If you don't plan on driving in the winter, you may not need the heater. What about a cool day when it is raining and you dont want to drive with your windows down and the windsheild starts fogging up?

But it's not really a question of "what is the need for the cowl area?", because the cowl vents are already there, so now they either need replaced (assuming they are bad), or plugged off. The only way to plug them is by welding sheet metal overtop of the existing vent ribs.

But, if your lower cowl vents are ok and water is not leaking into the floors from them, then you have nothing to worry about.