Left Lean
This crops up from time to time and it's no big deal , most of these trucks lead when new too , always the left front corner was about 1" lower .
Just Yesterday (2/3) I was up in Sta. Paula looking 'round the old things in the airport when I spied a all original '52 3100 from a block away and I nudged my brother , pointed to it and said " see ? that's the lean everyone always natters on about " .
If you take the cab off the frame and measure it . most of the time the frames are neither plumb nor square ~ these were designed and built to WORK as well as look pretty and since very single AD rig was sold long before it was built . GM ran the assembly plants flat out for two shifts and didn't mind the Quality Control very much , this is why they leak , squeak and have such terrible panel and door gaps .
If you're rich and patient , and want somethiong better than Generous Motors designed or built , it can be fixed , it just takes time and close attention to the details .