Are the RX7's body parts removable/replaceable? I was looking in the FSM and all it talks about is the bumper and various smaller parts. Did a search and people were talking about sawing and welding the body parts together? Looking at a RX7 with rust on the rocker panels, and think it might be cheaper to replace the whole panel from a junked car, then do body filler on it.

 

Only the obvious parts unbolt- fenders, doors, hatch/hood, bumper covers.
The rest is unibody construction, including the rockers.

Your problem is twofold- the rocker itself is a fairly critical structural member which is tied into the body with multiple layers of metal. A rust compromised rockers is bad enough, even worse is the way water, and thus, rust, likes to travel down seams, so you'd have to be suspicious of the doglegs (area right behind the bottom of the door) and maybe even the rear strut tower base.

Depending on the severity of the rust, I'd generally consider a bad rocker to almost rise to the level of "not worth it".
It is NOT an easy repair (for one thing, it determines how your door will fit) and once you start chasing the tin worm there's no telling how far into the car you'll have to go.

Unless you are performing all the work yourself- and are unnaturally attached to that particular chassis- such extensive repair easily outweighs the value of the car.

I'd consider another- less damaged- chassis.

 

Just to check, the painted part of the rocker panel is one of the layers? From looking at the body, it looks like the panel extends to the rear fender? If that part is not unboltable, then . Guess will have to skip that car.

 

I just took off a rear QUARTER for a guy,,Sawz-alled..I thought NO problem..Easy.RIGHT?
let ME tell you..it is NOT fun..
If you bent up the car that bad,that it requires you getting in there and bending the Reinforcement( Frame) Back..then I totally suggest you just Scrap the car and get another Shell..it is not worth the Time and effort,These cars are For Sale.EVERY day..somewhere.