I'll weigh in here as I just did this exact project on a 911 and have done many similar things on other cars...
Background: car is a 79 SC that we're building as an IROC inspired hotrod.
To answer a question in the thread, it is difficult to get the flare to butt nicely-but not impossible. Using the following I can usually get everything nice nice with about a 1/32" max gap. Using the following method you can do door bottoms, etc-any butt welded repair/modification.
Start by using some good measurements-i.e. torsion bar holes, rear lip above the valence, etc. to position the thing roughly.
Line it up, then make a line on the body where the new/repair panel overlaps-then take the flare off and grind off the paint about an inch or so from each side of the line.
ALSO grind off the undercoat on the bottom about the same amount. Take it down to bare, shiny metal. Use one of those resin discs like a Norton Beast-they do not thin the metal like grinding it. You DONT want to thin the metal, its thin enough already...and you'll burn through enough as is...
Anyway, that was your rough line just to get a ballpark idea of where the part will go. NOw you have bare metal to scribe your
actual cut line on-its much easier to see and make an accurate cut on bare metal than on painted metal.
Now, working with the flare, trim the edge so the edges are not wandering (from the stamping process) so they form a nice symetrical arch. Wandering lines are tough to weld accurately. Sometimes I do this by stretching a piece of tape along the edge and then tracing the tape edge...trim to the tape edge or the line you made, now you have a nice even edge...
Line it up as above but more accurately if you did the above just roughly, use some sheet metal screws/cleco's/rivets to hold it nicely tightly.
Scribe the quarter panel on the car thusly:
I use a super sharp scriber to get teh tip right down at the edge of the flare. Its also a good idea to make some hash marks that will allow you to index the flare once everything has been removed because you will then have no reference point.
Now remove the flare, use a 1/16" or thinner cut off wheel to make the slice on the OUTSIDE of the line you just scribed. If the line has a great curve, you'll want to use a smaller wheel as it will be easier to make follow the curve without making the cut choppy and innacurate-either that or just dont bury the wheel when you're cutting:
Now your flare is off-and holding it in place will be really tough as you cant clamp it too effectively, so its best to use your index marks and have an assistant who has not been drinking hold it in place while you tack weld it in a few areas.
Like has been said, you need to work slowly. Each tack weld, until you get them spaced about a half inch apart will need to be hand dollied to make sure the two adjoining panels have no offset between them.
Note the use of a small chunk of angle iron to make sure there is no offset between the panels-you could also use padded vise grips to accomplish the same thing.
Keep a clean small wire brush (toothbrush size) handy to clean your welds after you do them to make adjacent welds nice and clean.
(note-wheels are "Rollers" that are too narrow and I think we were taking the rear valence off because its hanging a bit low...)
Once you have 100 percent welded the flare, its time for grinding the welds. This is AT LEAST as important as your welding for determining how the end result is.
I grind welds thusly: I use the same cut off tool that I run an .045 cutoff wheel in, but I use an 1/8" so called cutoff wheel. This way you can grind JUST the weld and not the surrounding metal-its very controllable. I dont have a pic of this but its something that does not carry over well in a pic anyway. I hold the tool at a variety of angles to achieve the nice result I want-you just have to learn what works for you.
One thing I do is mark areas where the welds may not have penetrated well enough or where there were spots I missed. I then reweld these areas afterward and grind, etc.
ONce its all ground, you can hit it with a DA with some 80 grit to make it really pretty like this:
Sit back and marvel at the work you've done!