There are some differences between the monolever and GS paralever final drives. The brake drum and output flange diameters are the same, but the GS output flange is a lot shorter.
Here's how it looks when I mounted the GS wheel on the monolever drive. It can't be seen, but the brake shoes are hanging out into the gap. I figured it'd be best to try to fill up the gap somehow.
I planned to rebuild the wheel so I pulled out the hub to work on it.
Here's how it mounts up. I measured the gap as 22.5mm.
The hub has a center section that extends out. To cut it down I mounted the hub on a lathe with a four-jaw chuck
and used this indicator to get the hub centered.
Then used this indicator and a mallet to set the axial alignment.
After a few iterations of radial and axial alignment I could get the hub set. I cut off 21.5mm to leave a 1mm gap between the hub and final drive.
After I got the center section cut down I checked for interference by pushing the drive into the hub and and turning it around.
I found I needed to re-shape the uncut center section to make room for the brake springs as seen here. The outer edge of the monolever drum seals with two ridges, but the spoked paralever drum has only one outer ridge with the spoke nipples directly below. I trimmed a little off the outer edge of the hub to make some room, but there wasn't enough material there to cut off, and have some regrets now since it didn't solve the problem.
To get the clearance I mounted the drive on the table of a vertical mill using this setup and cut 2mm off the inner sealing ridge. I just used the X and Y hand wheels while watching the cut to manually feed the cutter around the circular path.
Even with all the cutting I still found the brake spring damper caused the spring to rub on the hub. The damper is really thick and causes the spring to bulge out. I'm not sure if it will rub when the brakes are properly adjusted, since the spring will move out a little when the brake is applied. I'll try a thinner damper if the spring rubs when the brake is setup.