Hi Gentlemen,
Forgive me please for jumping in to this October, 2019 thread now in June, 2021. Hopefully, you don't mind and hopefully, you both aren't dead of old age. I bought a 2008 535i as a body part donor car. By the way, I love that black wagon and I can really see why you want to do what you wrote about. Did you ever finish the project?

I also bought an e39 as a spare for a general parts donor car for my fully rebuilt e39. The trouble is that the donor car looks almost as nice as your black touring car. I hate to take parts off of it and I have not done that yet. This donor car suffered from the M62TU timing chain guide failure (by he way, I am an engineer and it would not be too difficult to put two internal idler pulleys and tap in for a bearing oil spray. Anyway, back to the reason why I am writing. Could I take the e60 turbo 6 cylinder and put it into the black e39? I also have questions about where to splice wires and how the two DME's are different. One would work the engine but the other would be needed for all of the gauges, door locks, fault codes etc. I can build custom motor mounts so that part is easy, as is the drive shaft. It is how to make the electrical work. Any ideas? I would need some pretty detailed responses if anyone out there can think of the electrical splices.