May not be the heater at all
ARCHIVED ROAD TEST
Despite its tire weaknesses, the Corvette is a superior automobile in all departments of handling. Like the man said, the machine is excellent, but the car's functional applications are limited. In addition to the aforementioned storage deficiencies, it is full of minor irritants—many of them the responsibility of the stylists, not the engineers. To begin with, limited interior space makes things very hot indeed, the glories of "Astro-Ventilation" notwithstanding. With that great lump of hot iron mounted a few inches on the other side of the firewall, BTUs pour over the driver and passenger in unpleasant quantities, even at cruising speeds.
It really does get hot in there.
I am not a fan of putting a stop valve in the heater line at all.
Check to see if the heat collar is in place over the transmission, if Chevrolet thought it was nessecary to put it in you know it was needed.
If it has been restored correctly it should not have any holes that let the hot air come in
, have a look and see.
You have a big block separated by 3 to 4 mms of fiberglass from you, not to mention that you are sitting over the exhaust pipe.
The privlage of owning a old classic.
Rosco