I think a 615 912 engine is a 220D, most likely a 71-73.
Your 616 912 engine is a better engine. In addition to the glowplug issue I think the 615 engine has a different oil filter location, no provisions for an A/C mount where your R-4 compressor is located in the 240D. The injection pumps are quite different as well. Your 240D is an automatic and the 615 engine is from a 4-speed so you will have to change the flywheel to the automatic version.
I would shy away from this one, even if you could get the 220D engine for nothing.
Good luck - Tim