1) That's a J-type. I'd wager that it failing to activate all the time is probably an indication that it is a little low on oil. That trans tended to weep a little oil, I had to add a half quart every once in a while. It was probably just the speedo cable seal, but I never got around to fixing it. Try adding a half quart (plain old cheap 30-wt motor oil) before doing anything else, see if it improves.

2) Typically, the flanges between the three sizes of driveshafts (thanks, Volvo!) don't work. Often, but not always, you can pull the flange off the trans/diff and swap it for another, but sometimes spline differences exist there.

3) AFAIK the rear section of the driveshaft is where the shorter wheelsbase of the 1800 makes up the difference. So it won't it on a 120/140. I'd suggest just getting the larger driveshaft extended a few inches at a driveline shop. The larger u-joints are just a bit tougher.

4) Welding the fittings on sounds like a good plan to me.

5) *Probably* but not sure. The 1800 has that Smiths speedo, no idea if the speedo end of the cable is the same or not. Worst case is you can still get the proper M41/122 cable from www.gcp.se.

6) I think the very latest production 122's all had cable clutches. I think (talk is cheap!) that putting one on an earlier 122 would just be a matter of using the clutch pedal from a cable car (I'm not sure if the 1800 pedals are the same length, *probably* are) and then drilling a hole in the pedal box up higher than the clutch M/C hole for the cable. (The M/C pushrod attaches below the pivot to get a push when the pedal is pushed, the cable mounts above the pivot to convert the push to a pull). Hydraulic clutches are presumably a tiny bit better, but the cable is at least dead simple.
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I'm JohnMc, and I approved this message.