Its going to be hard to straighten those frame rails out, even if you open the floor.  The metal is thick and there's not much room to swing a hammer.  You can move the crushed area downward, but the sides will likely require some attention too and this is basically impossible.  I can tell you that stud style pullers don't work on frame rails.  The metal is just too heavy to move that way.

I'd have to see them to say for sure what I'd do.  I had a lot of stuff like that on my '89 GT.  I fixed what I could without cutting anything apart.  I built some special pullers for a slide hammer to straighten out the rear torque boxes.  It looks a lot better than it did, but its not perfect.  I find a fresh coat of paint/undercoat, etc. makes it all look a lot better because its clean. 

If you elect to replace only a section of the frame rail make sure you use some fish plates inside the rail.  Go at least 6" past the cut each way and rosette/plug weld the fish plates to the existing rail and new section in 4-5 spots per each side of the cut.  Fish plates should be the same thickness as the frame rail. 

Depending how bad and where it is. I fix the dents in the 67 by making a box tubing piece that would fit snugly inside. I opened the ends up and forced it down through and heated where needed at the same time. It gives something to hammer against from topside down and the sides in. It's a lot of work and beer, but it also became part of the frame connectors later.