the steel is already as thin as possible. any thinner and it'd be see thru!!
you'd need to cut verical lines in the existing flared areas of the quarter, and add triangles of steel once the slits are opened up. then weld everything properly, and bury it in as little mud as possible
you'd also need to fabricate a inner wheelhouse, possibly tub it out.
plus the opening in the doorjamb, and the rear door skins would need the same treatment
if you search you might find one on here, I saw a custom steel widebody thread on here a year ago
pretty cool stuff, I do collision work
sometimes I get to do sheetmetal, custom fab stuff every once in a while. (doing a $200k '73 Yenko Camaro right now!)
I'd definately recommend $3000 worth of sheetmetal work than $3000 worth of shitty fiberglass...!
I attempted it, but couldn't find someone that could do the quality I wanted, so I abandoned the idea and went with a carbon fiber widebody. I did a bit of cutting to make the wheel wells bigger, but its welded together and the unibody is in tact...sort of.
Like bigshizzle said, an english wheel isn't really an option. You will have to cut straight up the wheel well and pretty much all the way across, measuring just the right amount of metal to add in place. If you can find a place that will do it for $3k and do a good job at it, go for it. I had no luck...look at my member journal and take a look.