Im a little late here butt will throw in a few things. However you do it i would butt the new panel to the old metal with welding magnets. (pretty cheap for a set at horbor freight) as opposed to flanges.. flanges you will have a pretty high build up of filler to blend it in to where it looks right. I am getting ready to do both sides of my 95 and got my panels for 88.00 a panel. I am going to butt weld my panels in with them being held flush to the existing metal with the magnets.

Be really sure you go down to metal far enough around the rust to get to good metal. If you dont you will be doing this again soon..(made that mistake when i didnt know what i was doing)..

Good luck and post pics of the truck when finished.

Rick