Frame rails aren't that bad, especially if you just have to do a partial section where a rat's nest was. If you're already doing the floor, it's the exact same process, just more spot welds. The only thing about frame rails is making sure you take plenty of measurements so that they end up back in the right place, and keeping the rest car straight while you've got the rail off. If it's just the rear half of the rear frame rail, say, from the shock mount crossmember back, the only thing that has to be even close to the right spot is the shackle mount for the rear spring. Of all the suspension points that could be involved that's the least critical, there's a lot of play in those shackles. And that's all that's back there. Probably the least critical section of frame rail in the entire car. I mean, it's not like it can't be misaligned badly enough to cause problems, it can, but you'd almost have to try to screw it up if it's only the rear half of the rail.
