It might have seemed like there was no progress on my own XS-Triple-Sidecar, but I did in fact get quite a few things done, but only in baby-steps. To be fair, I only realized how much I got done lately, when I compiled this post.
First step was to modify my early '77 frame to accept a stock XS750/850 oil-cooler. I made some steel bungs, tapped M8 to weld to the frame.
A bit of shrinkwrap should seal up the tach-cable for a while until I can get hold of a slightly longer tach-cable.
When removing the old exhaust most of the old studs (all but two) came out, so the threads in the head had to be recut and new studs cut from some left-over M8 all-thread bar.
Last part of the subframe-mods was to reinforce the rear-subframe by adding an extra lower brace that will take the sideloads of the rear mount as this proved to be a bit too flimsy during the first tests.
So with those issues out of the way, I am pretty convinced that now I can finally get to assembling the old girl and get her ready for a proper testride.