Due to the color I'm going to say Panel truck school bus. An ambulance would be white, red, red & white or Blue & white as was the practice back in the day. Yellow became a fire color in the 70s.

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}.