The first question I have is do you have everything you need between the 240D and 300D? I would think so but I'm far from knowledgeable. If so, it's just time and grunt and maybe a drill. I'd move the entire hidden side of the 300D to the 240D as assemblies to save time and energy - engine and tranny with oil cooler, prop shaft, rear subframe, exhaust... Sort out the wiring and heater plumbing as you go along. There are enough Euro 300Ds out there that you should find guidance on how to hook up the heater hoses.

Sixto
87 300D