The type that replace a core (freeze) plug in the side of the block are the most effective. These things are around 400Watts and will give a warm engine in about 2-3 hours after switching them on even in 20F below zero. Ones I have installed came with instructions detailing which core hole to install them into for the specific engine, and how to orient the heating element.

They have a limited life due to corrosion of the metal arms that hold them into the block holes, so it's a real bad idea to try to salvage a used one. It's gotta be near-impossible to get one out of a block without destroying those arms anyway. I had a new one fail after just 4 years, dump all coolant and warp a head. OTOH, the one in our 83 (supplied by the Volvo dlr) has been in there for over 20 yrs. As clbolt says, they're fairly cheap.

Most decent auto parts shops in cold weather regions should have them.
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Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD plus grocery-getter Dodge minivan, MGB, and numerous old motorcycles)