I do enjoy projects like this, and look forward to watching it come together. I also am new to sidecar things, but have a bit of knowledge about Harleys.
This shows how your sidecar was originally mounted to a big twin rigid frame, there was no swingarm. The lower SC mounting points are part of the bikes frame, two loops on the front down tubes, and two bosses on the axle plate. Rigid frames from 1936-1957, although there were many changes through the years, are the same basic platform.
Neat project, and good planning already!
@not-bob I bought it from an estate. The guy that had it planned to put it on a '72 FLH. I got two sets of mounts both one set for a rigid frame and one set for a swingarm. A '54 should also be a mechanical brake but someone in the past has replaced it with a hydraulic drum setup. Post '58 they connected to the actual swing arm itself. Sometime late 80s or early 90s the mounting point moved forward to the a mount at the swingarm pivot point. I have the fittings for when the mount was on the actual swing arm, but I don't know if the swing arm is the same size and shape and I don't know if the saddlebags would interfere with that placement. I'm planning to use the mounting at the swingarm pivot point.
Ok, always knowing I'm not the smartest guy in the room, I went looking. Couldn't grab any pics, but... what were they thinking? Yep, swingarm mounts. You have this project nailed I think. The swingarms were round up to 72'. My last Shovelhead was a 72', now I know what that empty boss in the swingarm was for! I'll just sit and watch from now on.
@panhead_kicker I have been reviewing some of your posts on a different forum as you were working through your connection challenges. I noticed you post discussing the 87114-93 vs 87114-93a.
Right now there I know where I can get a -93 (it's pricey but a guy has one). I haven't been able to find a -93a. Your issues with the -93a seem to have been caused by the exhaust Y-pipe. I have Rhinehart True Duals on my Road King so there shouldn't be any Y-pipe issues. My concern before buying the part is that it looks like it moves the mount point aft about an inch or so. Did you have any issues with the -93 contacting the bottom forward edge of the fiberglass side cover? I looked it up and the side cover for a '99 and '04 should be the same part (66670-97a).
The -93 did not interfere with anything on my 97 FLHP, but did require a bit of disassembly to reach its resting place.
If you're still debating heim VS ball joints, I've now dealt with both. The ball joints on my 69 FLH can be a little tricky getting lined up and started. The heim joints are easier in this regard, and on later iterations the heim screwed into a fitting that allowed toe-in adjustment without disconnecting. But that was more a function of the sidecar frame and fitting. Once you get it right, it's there to stay so this isn't such a big deal in my opinion.
Yes, the mounts are all expensive, and relatively scarce! A set of the front upper mounts left eBay a few months ago. The older mounts for rigid and swing arm mounts are probably a little cheaper as they are being reproduced and easily found. The 84 and later FLT stuff... well Vtwin says they produce, but I'm leery of their part numbers.