(1) My M41 has the selenoid on the left (driver's side). Which OD is that? J?
It is not actuating 100% of the time. Will a new selenoid fix it, or is a rebuild a better alternative. Where do I get a kit for that?

Yes, J. Yes, check/change the fluid level first. If it works most of the time, it's probably a simple fix.

(2) The output flange from the M41 is matched to the rather large diameter P1800 driveshaft. My Amazon uses the small diameter shaft. If I have my spare small-diameter foreshaft shortened, will it still bolt to the OD? Alternatively, if I upgrade to the larger diameter shaft (I already have a short foreshaft) Will the large diameter shaft bolt up to my P220 stock differential?

Alternatives that I see here... if you're having a section of driveshaft shortened, just have them use the large U-joint and yoke to fit the overdrive. Alternatively, find a late (maybe '72, definitely '73 ES) donor for the small flange and driveshaft.

(3) Are the rear driveshafts on the P1800 a different length than the rear shafts on Amazons? i.e. will I be able to use the rear section from the P1800 with my large diameter foreshaft?

Measure 'em. I don't know. See #2. My '66 wagon is fitted with a J-OD M41 and has small yokes and U-joints all the way. I do not know where the parts came from, but it does not appear that the driveshaft was ever shortened or modified. Perhaps from a '73 ES, maybe mix/match of 122/ES/140? I can get measurements from both my '66 wagon, and my '73 ES another time.. but not immediately. Might be helpful though.

I don't think it'll be so easy as using the large front section of driveshaft from the 1800 and the small rear section from the 122. Even if the splines work, I think you'll have issues with the carrier bearing and bushing.. try it and see though. I've been wrong before.

(4) Rear differentials... This has been covered in other forums. I'm still looking for a solution for the poor gear ratio in my wagon. I'm presently considering paying a machine shop to weld the 67's fittings to the P1800's rear axle so that it will bolt up to my Wagon. If you have a better solution, please suggest it. I know that a P1800 sedan axle will bolt to a 122 sedan. I have a wagon, it's no use to me.

With the overdrive, you might find the short gearing to be fine. And your speedometer won't be off if you keep the ratio. Save that 1800E rear end for another project... that is to say, a 122 sedan to which it is a bolt up proposition. If you must do the welding of a rear end to fit your wagon suspension, I'd go with one from a 140, which are much more common and will get you your disc brakes. It may seem like 140s are too wide, but the difference in track is only 1.4 inches. Of course, we're assuming you'd be using the 1800e front brakes/bolt pattern.

Save this project for another day. For now, work on getting the engine/transmission/driveshaft sorted. And again, you might find the ratio to be fine with the overdrive.

(5) Speedo Cable. Will a P1800 OD speedo cable be long enough for the amazon coversion?

You might find that the 122 cable will fit, if the 1800E is fitted with a right angle drive at the overdrive. If not, I think the 1800 cable will fit the 122 speedo head, but I'm not sure.

(6) Clutch Cable vs Hydraulic Clutch? The P1800 has a cable, the P220 has a Hydraulic clutch. I figure that I could probably just switch bellhousings to keep it hydraulic. However, is a cable-clutch better? Can it be converted with limited trouble? I'm thinking of eventually switching the brakes over to the dual triangular P1800 system, so getting that Clutch master cylinder off the firewall would simplify the whole thing.

Have you confirmed that your 1800E has a M41, and not a M410? If it's a M41, just stick with the hydraulic setup using your 122 bell housing. I see no advantage to the cable setup. If your O/D tranny is a M410, well then you're stuck with converting to a clutch. '69 and later 122s had cable clutches. Good luck finding one in the states though.

Keep in mind, you'll be wanting to use your 122 transmission cover and modify it to accept the 4th gear sensor. Unless you want the remote shifter, in which case you'll have to cut the tunnel.
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-Matt I ♥ my ♂