Hi,
Is the thingy in question not a heater, but a secondary engine cooling thingy to keep the van cool going up hills +5degrees. There are two modes hot, or very hot. When using this technique working quaterlights need to be functioning to avoid dehydration. The handbook also recommends drinking water a plenty, else know what that little lever on the engine is for!
Top tip, a hosepipe conviently fits on the input and output of the heater, i.e. a flush is possible in situ.
Top job on the welding!