Ditto Scott's suggestion...
There is a factory seam right at this point that was originally filled with lead.
This may be more prone to cracking on the panel/ burb than on pick-ups due to the extended body having to cope with the frame twisting over the entire legth of the vehicle.
What kind of shape are your body-mounts in ?
I have also observed that the early AD pick-up cabs (with the 3-point mounting) tend to crack in weird places: such as the dash-board, right above the steering column.
I guess this is why they redesigned the cab mounting to the four-point deal in '49(?)...
