Your motor may be fine, but if you have bad air flow, check for dirt in the heater core, or small aminal nest in the duct work.

Do you have the under dash heater of the unit under the hood?

1965 GMC 3500 Rescue in the Stovebolt Gallery

My Facebook Links:
Jolly Goodfellow.
the 6066 GMC Guy.
Jolly's Truck & Car Show Page.
Owner of ~ 1965 GMC 3500 Custom Rescue Squad {Black Built Bodies}.