One of my favorite block heaters to use on the older vehicles was the one that spliced into the lower rad hose. I used to use on a M38, and on a Manitoba winter morning, there was actually heat on startup. But when I went to buy one about 15 years back, I noticed a change. They had now included one of those little thermostatic switches on the heater to turn it off at a certain temperature. Now whether that was a conservation measure, or a safety measure, I knew I didn't want that style. Those little thermostatic discs seem to be the thing that will fail.
Re the flathead Fords, there were the blockheaters that would install in place of a headbolt.
The block heaters Phil talks about that go into the heater line are also a winner in my books. You can get them in wattage up to 1500W. I installed one years back on a HLVW wrecker that would be called on for crash duty in Moosejaw. The truck would start immediately on the coldest of days. Only problem was that you had to change the heater hose every 2 years. Even the silicone hoses I used for it would rot off form the heat, but it was a small price to pay for reliability.
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