I'm right in the middle of extensive rust repair on the rear fenderwells of my '87. This is not a small job AT ALL but the way the car is built, if you have soggy rear wells, your rear suspension structural integrity is basically shot. The wells are very stout and translate (directly) the forces from the struts and springs into the chassis. The thought of riveting patch panels in this area is insane and dangerous to say the very least. I'm glad you've got a welder now to do it right.
I will warn you though, the more rust you cut away, the more you'll find. If you don't remove every ounce of rust from the wells, they will just start rotting again. I have about twice as much rust to deal with as I orignially thought, and my car was already "gone through" by the previous owner. Of course...they did about an equivalent job to pop rivets and kitty hair bondo, and that made things worse than just leaving it alone.
When you weld near any of the original seams, you'll run into spattery poor welds because Toyota used a mastic sealer between each spot weld joint. To really do it right, you need to cut apart the panels which have rust going near seams, replace the metal on each part you've removed seperately, and then weld it back together like it was from the factory. Also, remove all of the undercoating in the areas you're working in. Much of my rust issue was hidden under this coating, and once removed, the panels started to just come apart in my hands. At first inspection they all looked just fine. Another trick I've been practicing is that I've been cutting out 1/4 of each fender well at a time right along the seams and at each spot weldThis makes for easy access, and also results in a job which I know is done right and will never give me problems.
Good luck....I've got probably 100 hours into 1 side so far and I'm ALMOST done....then on to the other side to start all over again.
SG in NE