Anyone ever have rust at the top of the rear spring mounts, where it joins the inner fender well?

I had passenger side at seatbelt:

Pretty much exactly like that. Is it repairable or not worth the trouble?

Completely repairable... I've got pics somewhere... but I took mine down to bare body. I had rust at pass side rear spring mount/seatbelt mnt. Also had rust at rear fender wells. I cut and ground everything out down to good metal. (note, while I was grinding out at intersection of of fender well a and floor pan, I was not holding tightly and grinder wheel pinched, bounced off body and smashed into my face- I was was very lucky as it hit me just above my teeth and at edge of my nose. Now have a gnarly scar there... inner repairs I used 12 gauge sheet and got fender well pieces from autoricambi. I used a spot welder and mig....

Got a little flux core mig welder. The spot welder is my next tool investment. Glad to hear it's fixable. I've done the front strut towers. I was hoping that this wasn't going to be the end.

Found some pics:

Nice. Doesn't look too bad. Like the spot welder. Is that how you were joining two panels to fix the rear quarter panel?

Yes. I used the spot welder and offset bender to fit all parts and then I ran continuos bead along joints with mig.