Frame Re-Enforcement
Your ladder-bar problem does not sound much different than problems associated with heavy snowplowing. Here is the fix I came up with and have applied to dozens of trucks.
Cut 4 (2 per side) pieces of 1/4" plate at least 24" long (inside & outside) to match the shape and sandwitch the frame. Drill out the plates to go over existing rivets/bolts to obtain a flush mount. Unfasten brake/fuel lines from frame, they will be refastened to the new plate-sandwitch. In the case of your ladder bars drill out the FRAME hole 3/8" larger that the bolts being used for the ladder bars, this gives 3/16" clearance between bolts and frame. Drill the plates to the bolt size. This moves the stress from the ladder bar bolts to the plates instead of the frame. The plates are fasten to the frame with at least 6 bolts (3 on top 3 on bottom 12" apart).
I'm not sure how your mounting the Ladder bars but you should get the general idea of transfering the stresses to plates vice frame.
Hope my 2 cents helps
-Matt