MONTE CARLO southern state guys - is this considered normal rust?

It depends on what part of the south and the circumstances. If you're talking humid Gulf Coast and the car has been sitting outside (parked up, not being driven), that rust isn't unusual. Cars are designed where when you drive them, air circulates in the doors and other inner panels and drys them out. When the car sits up, that doesn't happen.

No matter what, if water is infiltrating the interior and the carpet and padding are staying wet, the floors are going to rust.

Out in the southwest, it's so dry, it doesn't matter if the water gets inside the car, but the paint and interior get baked from the sun.

For perspective, my brother's dailey driver is a 22 year old GMC that doesn't have a spec of rust. It is parked outside on a concrete driveway and it gets driven regularly. Park it up on dirt or grass for 5 years, and it likely would develop rust. Oh, this is in Houston.

It seems a little excessive for a southwest car. It was probably brought here from somewhere else and “sold as” a southwest car.

I would say since you said the frame was off at some point and has POR 15 on it along with new body bushings then you are ahead of the game and I would just do the rust repair and move forward with the project.
IMO welding in floorboards are a relatively easy fix.

My brother's 78 Z28 had pin holes in the floor boards, a rusted out lower rear quarter and trunk drop off, and rust damage in the rear window channel. The car has been in Houston since at least 1985. It sat under a carport from 1990-1998 and then was garaged from 1998 to 2006. We restored it from 2006 to 2017. The inner structure was pretty sound but most, if not all of the rust damage was from sitting around.

Texas82GP said:

My brother's 78 Z28 had pin holes in the floor boards, a rusted out lower rear quarter and trunk drop off, and rust damage in the rear window channel. The car has been in Houston since at least 1985. It sat under a carport from 1990-1998 and then was garaged from 1998 to 2006. We restored it from 2006 to 2017. The inner structure was pretty sound but most, if not all of the rust damage was from sitting around.

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Garages if not properly ventilated will trap humidity inside and can cause worse rust damage then if a car was parked outside.

If a car was ever exposed to salt and never washed off the underbody, it can cause extensive damage. Even if it was exposed just once and never washed can do it.

Of the 5 g bodies I have owned

3 were sold new in the upper Midwest (WI and ND) and stayed or migrated to rural MN/ND
2 were sold new in Dallas area and made it to MN sometime in the 90's

Only 1 was rusty enough that it was parted out due to no title, driveline and it was a factory v6 bench supreme cutlass (in retrospect it wasn't that bad, I was just used to cars in much better shape)- Gravel road ND farm car
1 had light floor pinhole patches due to a leaking rear window and HVAC box (TX car)
1 had some light surface rust on the floors (ND car)
1 had light door seam rust and a small spot in the rear frame rail (WI car, sent to northern MN in the 90's)
1 was spotless (TX car that was sent to Northern MN in the 90's and dairy barn stored for 15 years

Granted I bought all of these 10-15 years ago when G bodies were still 'upcoming' especially in my part of the country where most just go to the dirt track to roundy-round. ND and rural northern MN is also historically too cold for salt to work so stuff didn't rust out the same as NE or traditional rust belt car areas.

Seeing rust creep deep into the rockers in my experience means much of the other parts of the car have worse rust. Frames, window openings, etc.

It's possible the rear window could have been leaking and that rust is only on the rear seat pan, in that case its worth fixing if the rest of the car is clean. Otherwise ish.....

Of the 8 g-bodys I've had over the years (5x Malibu's; 3x El Camino's), the ones from the local Dallas/Ft Worth area have only had floor pan rust in the pass side front footwell from A/C box seal leaks. Mostly pinhole stuff @ most. The worst one was from one of the Elky's that came from the Missouri area to DFW.

If I were living up North/East, I would def scour Texas, Arkansas, New Mexico, & Arizona for relatively rust-free tin.

My 81 TA was from TX, no rust but paint and interior needed redone. My S-10 (used) looked great at sale, within months rust was coming through. Turned out it came from Northern IL.

In "younger days" and living near Chicago I bought a 79 GMC "from Texas" which was one of it's sale points. You would think "rust free", meant rust free. However, that vehicle came from near the Gulf and the whole underside wasn't any better than any other Chicago vehicle exposed to heavy use of salt.

In Arizona now, plenty of rust free cars, but also some here from Northern states due to people moving here.

Your rust looks like road salt issue. Or you are having an issue of water coming in and pooling. The only fix is replace bad areas, and at today's prices it is most likely better to repair, than to find another Monte which is getting harder and harder to do.

What year is your Monte? There is an 85 down here that would make a good doner.