Wow, you've already made a lot of progress! I used that same method to pull the drivetrains out of mine when I did the swap. Pretty efficient way of getting it done.
Nice looking color on your 740. Sounds like it should be a very nice setup once done. Yes the clusters are completely different from the beveled later type to the earlier rectangular units. Your best luck to get a tach and glow plug light that works in the rectangular cluster might be to get a junkyard cluster out of a diesel built before 1991 (or before 1988 if it is a 760 -- 760 got that later dash earlier than the rest of the series, the others didn't get it until '91). Could also use a Dakota Digital adapter box to make the gasser tach work -- I have used them before and they work well, but they're spendy (plus shipping from US), and then the redline is in the wrong place anyway.
I have a spare tach from the earlier style cluster I could sell you that would drop into yours, but the shipping cost might be killer and you can probably find a whole diesel cluster easily enough close to you anyway in a junkyard somewhere. That way you'll get the warning light strip with the GP light in it, too. Seems to me that would be the easiest option. I wouldn't want to think about swapping the whole later dash in -- IIRC the climate control setup is different on those later cars too, would be a lot of work to get a tach installed....
Be sure to hang onto those E-code headlight assemblies from the 940 -- those fetch good money if you want to sell them to someone in the US! Much better than the lights those came with on our side of the pond....