That frame rail will need to be addressed before one jumps over any train tracks.
FMJ's are getting a bit harder to find but they are still out there. Most M-bodies do not have the "factory installed rust option" that most F-bodies were built with. Matter of fact, I would say that most M-body cars have done really well overall through the years with the rust issue. Also, the '88-89 M's also use galvanized steel for most of the body.
The front floor pan and frame rails are the same for most all of the F & M-body cars with the exception of a few miscellaneous welded-on brackets that can be removed and added back on.
The rear floor pan length depends on if a 2 or 4-door F or M-body, but a 4-door rear floor pan can be cut down if needed for a 2-door.
Using a spot-weld removing drill bit, and some time, you can remove a floor pan and frame rails from a newer M to fix your car.
For a while, a company was making a steel sleeve to weld on over your existing frame rail. I think called a "safety cap" or something to that effect, but they making 'em quit a while back.
There is someone here who might also have frame rails and possibly, a floor pan for sale (not me) so you might make a post in the parts wanted section and see who bites.
It is a bit of work, but several people have successfully replaced the floor pan(s) and frame rails - so it can be done. You might even to find threads here about it with step-by-step pictures.
I don't have the rust issues you have, but I do plan on doing some spot weld removing to upgrade my radiator core support (going from 18" to 26" wide radiator), A/C firewall (my car was a non-A/C car) and a larger trunk pan (much larger spare tire area).
BudW