If you're going to take remove any sizable pieces I would built a jig like Jim mentioned just to make sure everything lines up when you're done. But keep in mind that even a jig won't hold a piece true if it's welded incorrectly, warped is warped.
Without any major structural changes though a jig isn't mandatory. I fully seam welded and reinforced the K's that are in my Dart and my Duster without a jig and didn't have any issues. That doesn't mean you CAN'T warp a K member doing that, I'm sure you probably could. But if you're decently versed in welding then you should have an idea of how to weld on something like a K member without warping it. They're pretty heavy items.
As for the factory welding on the stock K-members, yikes. The welding on both of mine was seriously ugly. On the Duster the steering mount welds had cracked, allowing the steering mount to flex back and forth when the wheels were turned. It had a ton of play in the steering (obviously) that I originally thought was in the box, til I had a helper turn the wheel back and forth and watched the steering box flop around. :violent1: While the K member was true, I had to do a decent amount of work to get the steering mount squared away before I started reinforcing.
I'm sure seam welding and reinforcing/gusseting the K's stiffens them up, though I'm not sure it's something I'd ever notice driving my car (as long as nothing broke!). But after seeing the welds on the ones I've done so far, I'll be seam welding and reinforcing any K member that comes out of one of my cars before it goes back in. Obviously it's not a "must-do" kind of thing if everything is working, but for me if I pull one out it'll go back in fully seam welded and reinforced, regardless of the intended use of the car. Just piece of mind for me.
. so my mind has been somewhere else. 
