Well, after much procrastination, I finally decided which direction to go.
I decided to mount the solid axle tube type rear unit from the 250 into my 300 Fourtrax.
I was going to just make a flat steel plate to weld to the axle tube, like the one pictured in the 1st post. But decided to use part of the axle tube from the 300.
I cut the section of the 300 axle tube off that originally bolted to the swingarm, then sliced it in half.
Since it was a larger diameter than the 250 axle housing, I cut a wider section off of the same tube to act as a shim/spacer. I then removed about 1/2", before i sliced it into 2 halves, so i could put the pieces in the vice and make smaller diameter halves to fit.
Then I bolted the differential to the swingarm, and set the lower pieces in place. I just needed enough of a lip to weld the brackets to the sleeve/spacers, and the sleeve/spacers to the axle tube.
Same w/ the top halves. It has no choice but to line up!
It's been "dry-fitted" now, so clean up the area to be welded, re-fit, and bolt into place.
I then just tacked everything in place, removed the unit, welded, painted, and reinstalled.
The finished project, a 250 Fourtrax rear differential w/ solid axle tubes mounted on a 300 4WD Fourtrax