Yes, I sanded and painted both door sills, alternating coats between one side and the other. Only about 30 minutes of effort in total, but drying time between coats made it a 6 hour job. The rusted areas under the center console are easy to access (once the console is removed, which is kind of a pain), so I'll probably fix those next Summer. Under the instrument panel, I'll just have to live with until it threatens the integrity of the structure.
The '98 in the wrecking yard was a convertible that had been totaled. It no longer had an engine, transmission, steering wheel, top, hood, trunk lid or any of the interior trim. It sill had an instrument panel, but no gauges. This car may have been completely exposed to the weather for months or years, so I considered it a worst-case example.
Do the 80's Mustangs have similar rust problems? I don't think so. I subscribe to Mustang Monthly, which is directed at those interested in restoration, and contains lots of advertizing for old Mustang parts. Soft parts, gas tanks, battery trays, door shells, and floor pans seem to be the big sellers. I have not seen any ads for interior support metal pieces.
The current obsession to save pennies of cost on every car is fairly recent. Ford will always do what is best for Ford. I use to sell them parts and I know how they think and operate (GM and Chrysler are not much different).