MOVING WHEELWELLS 101
If you need to get more tire under your stock wheelwells, this is the easiest way to move them inboard, no matter what car you have. Retaining as much of the original well as possible greatly minimizes the work and cutting involved. Re-using the stock inner well also makes it easier to re-weld in place as well as re-fitting other brackets that are attached to the wells. The only advantage to completely replacing the entire well is the fact that aftermarket ones will be square vertically, thus allowing a taller tire. The stock wells will accomodate up to around 29.5, though the stock front edge of the quarter is the limiting factor. 
Here are the stock wells in my '69 Swinger before any cutting. Before changing anything, make a cardboard pattern so your floor cut will be the same shape as the stock well. Also notice the location of the trunk bracket in relation to the "shelf" on the well. Coincidence? 
In the front you'll have to drill the spot welds holding the face of the package tray. You can then just bend it out of the way and hammer it back down to match the new well later on. It's also a good idea to remove the gas tank, filler, fuel line, and brake line where it runs through the well. 		       
         
After cutting the trunk bracket off, cut up over the center of the well, then around the bottom edge. stay close to the well around the front and back as this area wont be trimmed much. Some places are easier to reach from inside the wheelwell. Once it's out, drill the spot welds holding the piece or trunk bracket on, as well as the strip from the trunk floor. It's also easier to remove the undercoating now for welding. It's dirty job when it's above your head. Once this is done its ready to re-install.

 

Now lay your cardboard pattern over the trunk floor with the middle area flush with the framerail. Most of this pattern is straight. It's only the ends that curve around, so alot of this cut will be against the rail. If there are any doubts on the curves, leave extra metal and trim them to fit later. The finished cut shows how far the trunk bracket is over the new well location.

 

 

After some trimming by trial and error, you'll see that the trunk bracket now rests nicely on the "shelf" on the wheelwell. The diagonal brace also needed to be slightly trimmed on the bottom side to clear the well. You couldn't see it in the pic above, but I moved the cut over near the bottom to leave the seat bracket in the original location...one less measurement. You also lose the outer mount for the seat belt. This plate could be moved though, as it's not important to have in an exact location.
Once you're satisfied with the fit tack the well in place and look it over once again. One thing to watch is that you dont rotate it fore or aft. make sure the top is even front to back with the top of the outer half. Sorry for the blurry pic...couldn't hold still:) If everything looks good, weld the trunk bracket on, then the package tray face plate. This will keep the well in place for the rest of the job.
      
Finish welding up the inner well. Spot welds a half to an inch aparts are fine, although it wouldn't hurt to weld it solid. Next step is to cut a strip of metal to fill the gap between the two halves. In my case, this gap was 2 3/4". Dont cut one long strip and expect to weld it right up. Cut shorter pieces like 10" or so and weld them in from one end to the other. The gap isnt going to be perfectly straight so these pieces will have to be hammered slightly to fit. Tack an end and work your way to the other end. The best way to weld this up is from inside the well. Its not a fun job, but the results are worth the effort. Once you're finished, hammer any spots flat, and any other finish work you may want to do, and you're done.
      
Here's the finished well. I chose to use seam sealer at this point, and then just paint it. It looks factory to the untrained eye, and is just as reliable. Even to a trained eye, a non-original, non-show car doesnt need to be massaged more than this. If you want though, you can weld it up and use some filler to smooth the well out to look like the original solid piece.
All finished. That wasn't so bad. Actually, I spent about 10 hours a side, if you count all the coffee breaks:) Not too bad to fit 12.5's in there.I hope these pages help some of you out who were thinking of doing this work. Any comments of course are welcome:)