I cannot speak specifically on an e-body, 440, or hidden a/c.
I can speak of adding a/c to a convertible living in texas.

I used the classic auto air system in a 65 coronet convertible.
The system functions well, cooling pretty well based on air temp differences. 30 degrees as I recall last time I checked.

With the top up, windows up, in 100 degree weather, it keeps me from sweating, but just barely. It never kept me cool.

with the top down, you don't even feel a cool breeze.

This is probably the worst money I ever spent.

If I take the car out, I want the top down.
IF it is too hot for that, I don't drive it.

you may have different motivation for driving yours, but wanted to give you an opinion.

It is not the same as a modern car, and even those don't move enough air with the windows open. I checked, incase it was just a bad system thinking it wasn't moving as much air.
not having on old a/c system to try, I can only go by memory thinking the original a/c I had in a 66 blew colder air and moved more of it, but memory is a tricky thing. for all I know it was just as bad.