Block heaters are usually in a frost/freeze plug hole. There is a good possibility it will leak if its that old. I had the block heater on my 81 Malibu corrode and blow out one day while driving. If that truck were mine I would replace it with a plug.
If its hard to get at, as was the one on my Malibu (under the exhaust manifold), you can put in an expandable rubber plug which tightens with a nut/bolt. Easy install and mine has been in for about 6 years on a daily driver.
