I would try blocking the cab level with the frame and centered, and then install the shackles, using all the slop in the holes to line them up. Maybe push the frame portion of the brackets up as necessary. Then tighten all the bolts. You could cheat a bit and put shims between the frame and shackles to move the rear of the cab to center it.

I haven't tackled that task yet, but can understand how it would be frustrating. Good Luck and let us know how it works out.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.