As you read through the linked thread, keep in mind the word "channeled".
The Caprice frame is much wider through the middle of the car and the floor isn't going to match the frame at all.
If you just set a 55-57 body down on a Caprice frame, it will not drop down far enough. You will see the frame below the rockers, and that means the car will sit high and that's an UGLY way to have one sit high even if you like a car that sits high.
So what you have to do to make all this work and hide the frame is to ditch most if not all of the floor, channel it over the frame and build some body mounts on your new or seriously modified floor. When you do this, you're going to have problems with the seats fitting in the car as well as other things, because the floor is going to be really high at the door sills/ends of the seat.
After that you're going to have to build a radiator mount, and figure out how to make the radiator support and inner fenders fit the steering box and shaft. Then you're going to have to build something to mount the bumpers.
This is all a tremendous amount of work. The guy that had the car in the link was going to have to do 80% of that anyway to get a floor back in the car. You may have more to work with than he did.
I'd never recommend that anyone do this. But if you do, you need to have your eyes open.