I don't think location is all that important, ie any frost plug that is easily accesible, as I said it was just over the oil filter on mine. The heater will heat the coolant which in turn heats up the entire engine block
Have a look around the engine block for other frost plugs that might be easier to access and simply measure the diameter of the hole and order accordingly.
The auto parts should be able to provide just about any size heater based on the hole diameter.
In my case they actually had a referance to the Mitsu K3 engine (and possibly your K4 as well)
I could not find another accessible location (well easier than behind the filter).
Do for sure go for the higher wattage unit that you can get.
Good luck!