an aftermarket cable controlled valve can be had at most any parts store that fits into the water supply line under the hood
like this - any future leaks don't happen inside and you can still cut down or cut off hot water flow to the core, but my experience is the usual heaters in these old trucks are easily temperature controlled using the fan speed, if you really need heat, full blast is barely enough, the main time you'd use the water valve is to shut it off in the summer
Bill
