Question about my just purchased '68 C10

[rustytruck50]

I have a cab sagging dilema. After lots of measuring, I found that the cab support on the drivers rear has dropped about an inch, leaving the cab really close to the box on that side. A previous attempt at cab corner replacement omitted the rear corner filler panels on both sides that close the cab from the road. Long story short, is there a good temporary fix, like shimming that corner, that would be ok until those fillers were put back? Any help would be greatly appreciated!