Well, after booking an appointment with the 4Runner specialty shop here in town, they didn't have any openings until next month, so I had some time to think about this job.
I rummaged through my garage and found an adapter for the wire cup attachment for my angle grinder. It had an adapter included that was a 5/16"-10 to M10-1.25.
So I punched out all the tack-welded nuts from within the frame, widened the hole for the gas tank strap bolt, and inserted the adapter in there to act as the nut.
I welded that and two nuts for the gas tank skid plate to the frame as well. There was no way I was going to fish the nuts inside the frame with whatever tool (coat hanger, magnetic tool, etc...) since there is so much crud/sand in there, plus the fact that I wouldn't be able to weld them in place.
Since they're attached to the exterior of the frame, it's going in create a small gap when the strap & skid plate are attached, but it's the best I can do to get the items back into place.
Wire cup adapter for the win! Cancelled my appointment with the shop and saved $500 in labour.
Welding overhead, next to the gas tank itself was a little strenuous.