I've already made a couple of changes on this project. The first is I'm going to use a YX600 Radian swingarm, which is a popular swap on the XS650. It is almost a bolt-in. The pivot is a direct fit using the XS650 pivot bolt.
Here it is compared to the stock '75 XS650 swingarm. It is slightly longer, uses a 17mm axle compared to the 20mm XS650, and the shock mounts are farther back and a little wider.
I'll need to drill out the adjusters and enlarge the axle slots to use a 20mm axle. The XS650 axle is too short, since the adjusters are wider on the Radian arm, so I'll need to get a longer one. The shock mounts will need moved inboard about a 1/4" inch on each side.
I did a quick mock-up of the arm on the bike. I like it. It reminds me of the square swingarms on the '68-'70 DT1, so fits the look I'm after, and is stiffer than the stock XS arm. When I move the shock mounts in I'm going to keep them in the same location front-to-back as I like the shock angle better than if I moved them forward to the stock XS position.
The other change is that I decided to completely disassemble the bike to sand blast and paint the frame rather than just touch up the areas I'd worked on. I'd removed the old coil mount, side stand mount, four center stand mounts, and welded on two exhaust tabs, new coil mount, new side stand mount, new rear tank mount, moved the front two tank mounts, new seat hinges and latch mount, four new skid plate mounts, and some other things. It'll take about the same amount of time, and I'll be much happier with the results if I do the entire frame. The easy way to remove the engine is to take off the engine mounts, but leave the two long bottom bolts in place, lay the bike on the clutch side with something to support the engine, pull the two long bolts, and then just lift the frame off the engine. I use a couple of small airplane tires with a towel over them to support the engine. Do the same method reversed for putting the engine in the frame. I'll probably remove the passenger peg/exhaust mounts. The right side mount is also where the rear brake switch and brake tension spring mount, so I'll look at either leaving the part they mount to or relocation them.