The only reason I would caution against that is now your replacement panel going in is undersized. Welding=heat=shrinking. Any intention/consequence of leaving a gap is only adding to the possibility of more shrinking, or panel movement. A low crown panel will tend to lose it's shape quickly with any shrinking, so a gap only allows the panel to pull together that much quicker, making more work to address low areas or a loose oil canned panel. If you've ever seen a panel close up tight on those butt weld panel clamps, you have seen this effect in action. If the panel is such that the cut goes straight across, and once fitted up you can slide the panels apart to compensate for the cutter thickness, then in that case perhaps it would work without any ill effect.
Here's a thread with some good content on panel fitment, tacking in place, planishing. Pay attention to the replies by Randy Ferguson (his coachbuilding is well known for Willy's reproductions). A couple of the other responses may contain less than ideal methods, so I'd say to focus on Randy's posts.
http://metalmeet.com/forum/showthread.php?t=798
