I think most, if not all, block heaters are installed by removing a freeze plug in the block and replacing the plug with the block heater. I haven't done it myself, but I don't think it's too difficult. Large plugs are removed by drilling out the centre and removing with a slide hammer or similar tool. Small plugs can be removed by prying them out with a punch or chisel. Of course, care should be taken when prying or banging on any cast part such as an engine block. Castings break easily, and they are expensive to replace.
Other type heaters are probably easier to install, but block heaters are probably preferable. I think block heaters are better at keeping cylinder walls warm than other heater types. Warm cylinder walls help diesels to start in cold weather. Perhaps the 're-circulating' types installed in rad lines accomplish the same thing, but everybody around here uses block heaters on everything.
Magnetic oil pan heaters may keep the oil warm, but do little for engine block temperature. I use a 0W-30 engine oil, which is probably better at keeping oil thin in cold weather than an oil pan heater. I do use several magnetic heaters on the transmission case, even though my dealer says they don't actually do much. The dealer said that using a good winter hydraulic oil is better than heaters.