And i guess its cold rolled steel ay, 1.2-1.4mm i think
Hi Bart. If you were to make the top section with a 20mm folded lip at the top you could use the shrinker to pull some curve into the panel through the lip. The only problem with that though is it will make the top edge of the quarter curved, and it needs to be straight. If you can get access to an english wheel use a fairly flat bottom roller and put some crown in it, but it wont need a lot. You'll be able to get some shape into the panel when you weld it to the lower half of the panel as the lower half should have the same curve in it. Does that make sense? I don't think I'd use a shot bag and hammer to get the curve in, as it's such a mild curve I think a shot bag will stretch the metal too much by the time it's planished smooth.
It looks like there is only a small curve from top to botom of that top section of the rear guard. I'd probably just bend that by hand over a gas bottle to start. If there isn't much curve front to back, then the english wheel will probably put too much shape in, as it curves the metal both ways. I really do suck at explaining things.
Just give me your LJ and I'll fix the Ponti for you : )
Oh, and regarding wheel tubs, I only had to stretch the bonnevilles tubs 100mm to be able to fit 20x16s, so they're plenty big : )