There should be a valve in one of the heater hoses, probably under the hood. It's possible your valve corroded so much it became un-usable and was removed by a previous owner. Universal valves are available at auto parts stores.

The reason there is no mention of a heater in the assembly manual is heaters were a dealer installed or after market item in those days.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.