You can find a cut off valve on eBay. You can either find one controlled by a cable or you can get a 100% manual one which requires you to open the hood and screw it clockwise to cut off or counterclockwise to open. I think I'd get a bucket and drain the coolant out of the radiator & block to prevent leakage. Good luck. Heaters this old are notorious for getting thin, cracking and leaking. The good news is you can make one out of 1/4" copper tubing wound into a spiral and a couple of 1/2 to 1/4 copper adapters (assuming yours is a circular coil). If you need information on how I did that, just let me know.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end