Great job! Two running engines, a usable main clutch, and one steering clutch means it's mobile, at least for fixing. I agree with Mike, it seems like that right side clutch needs to be pulled:
- split track at master link
- undo pivot shaft nuts/bolts on right side main cap underneath the transmission. I think the dowel is on the left side case so you might not have to do anything further regarding the pivot shaft
- jack tractor up on that side, or the entire back end so's the sprocket clears the rail and the pivot shaft sags a bit from the final assembly
- undo the 3/8 nuts/bolts that hold the final drive assembly on to the main transmission case
- using an engine hoist or your overhead crane, lift, wiggle, tap, swear at, and generally finess the assembly off the main case. There are a couple of binding points - unsticking the assembly from the gasket from the main transmission case, sliding the assembly off the two dowels, sliding the outer/brake hub that is part of the final assembly off the clutch plate "fingers", sliding the sprocket shaft inner bearing out from the main transmission case boss it sits in, and sliding the assembly off the pivot shaft. So, it will seem like it's stuck but it's just a whole set of things combined to make it seem that way. My technique was to use a paint scraper to go around splitting the gasket (Cat has new), tapping the dowels while pulling the assembly to free them them up in the holes, fully removing the pivot shaft but that's a whole extra piece of work, maintaining a steady outward pull while tapping and wiggling etc. Once it was a bit loose, I also cocked the assembly over, and then turned the sprocket/gears so that it kind of worked the clutch plate in the splines to further loosen up rust etc. Took me a couple of days..... Of course now that everything is new and well-oiled, I imagine that it would slip off like butter on toast : )

From my experience with my 5U, that amount of rust likely means the bearings are toast. Once the bearing gives enough, components will break. I think it's a better prospect to pull it apart, replace the clutch components and bearings, then it'll be safe and it will work properly. It's really not that bad of a job but I hear you on the need to prioritize other things - that's why my 5U is still not finished after years! : )