Ya gotta remember, these trucks were tough as nails and were used hard. My 3800 has those cracks and I know exactly how they got there. When I got the truck, the rubber around the column was entirely missing and the rattling drove me nuts. So I cut several layers of inner tube and clamped the steering column down tight. Then coming out of the woods one day, over very rough ground, loaded high, wide and heavy with wet shake bolts, I hear a loud pop from the dashboard and the rest, as they say is history.
I'd beefed up the front springs with an extra leaf and the heaves in the ground were beyond the limit of the suspension travel. The rear was right down on the rubber pads, so the frame twisting was all the suspension left. So, the dashboard couldn't stand the strain. Years later, neither could I so I took the extra leaves out of the front springs and added shocks to the rear. Vast improvement. The cracks are still there.
