My Helmet is off to you Lukas great work, great Journey, great posting.
I just stumble on the posting since I also have a 640 frame looking for a engine and you are saving me a lot of the home work already, in any case I have a few question for you before I continue on this "ShoeHorning" journey.
In my head at least seems logical to start by "Affixing" the counter shaft sprocket location as close to ideal since that function is a FACT, a.k.a. it need to be where it needs to be, the rest of the brackets can be fabricated in one way or another, but the chain line and the path of moment of the chain can not be tamper, so why are you trying to mount the engine with out having it already attach (temporary) to the frame....??
Anyway that is my question for you...
Oh also I have a great advantage in terms of engine selection since I'm going to use the chassis for a sidecar so i can run a engine that is not exactly in center (Triple, four)
Thanks again for the wonderful read.