I had to do this same repair on one of my fenders. I can't remember if I wedged a board against it or used a ratchet strap, but it wasn't hard to hold the fender in the right position for welding. I ground the cracked part out a bit (dremel), welded it up, ground that flat (grinder), added another layer of steel and welded that on. It's sheet metal, so if the welder can't go low enough on current, just keep doing a series of 2 second tack welds.
If this is a GM fender, it's much better to repair it than swap to an aftermarket fender. Those don't fit right and are less durable. A well repaired GM fender will probably last longer than a new aftermarket fender. Even if you have access to a nice GM fender, this repair is cheaper and easier than painting.