OK, here's the 3800 front bumper. The bumper itself is a shade under 1/4", like .244". The "bumper bracket" for lack of a better term is 3/8" x 2 1/4" thick, and like OldSub said bolts to some cast pieces on the front of the frame horns.
Whole bumper:
And closer up:
And here you can see the cast pieces they bolt to:
Dan, what should I measure to determine if the fenders are larger, the wheel opening straight across (horizontal to ground)?