Little update:

Ordered a spare stock swingarm to use if needed.
The 83 GS550L swingarm has an inside distance of about 4 1/2in. Our stockers are closer to 6 7/8. Clearly not going to be a plug and play option by any stretch.

I'm brainstorming solutions for this that include strengthening the frame while accommodating the GS swingarm and linkages but going to backburner it for now. This is quickly approaching frame construction which I didn't want to get neck deep into just yet.

More to follow down the line.