The FAM calls for two flavors of shims above the frame, then a shim and washer below the frame hole. I think it's important to have some flexibility both above and below the frame. The split lockwasher might loosen over time with the two flexible shims sandwiching the frame mount. I would think that a self-locking nut would be better. Some of the guys have recommended tire sidewall or conveyor belt material for the flexible shims.

I've actually just been thinking about that cab mount and was wondering about drilling the frame hole larger and use a stepped flexible washer or a spacer around the bolt so it could be tightened completely. or Maybe a polyurethane bushing to fill the space between the hole and the bolt. My frame shows signs of the bolts rubbing the holes oblong. That would likely add to the cacophony of squeaks going down the road.

Sorry I can't help more. There are likely additional shims needed between the body and frame to put the body at the correct height above the frame.

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.