Hi Doug, welcome..!
Originally, there is/was a shutoff valve in the heater hose that runs along the valve cover.
It may be defective.
The flaps that re-direct the heat inside the heater have foam seals that go away pretty fast.
The heater has only 4 bolts holding it in place.., along with a flex tube on the right side, 2 defrost ducts,
& a couple control connections (push/pull wire). Usually req. the radio & ash tray removed to R&R heater.
Not really a hard job.., just awkward to get hands in there, etc.
You can fab. something to block off the air inlet..., it's on the pass side under the hood, on top of the cowl.
However, if water still flows thru the heater core.., it still radiates heat.
You can loop the heater hose so no water flows to the heater core.
Lastly.., be sure there's no holes/leaks in the firewall, allowing eng. heat (& fumes) in.
