The problem is that the only currently availible solid axle shims are too wide for the older trucks. The narrowest that I've seen is 2 1/2", the spring seats on the old Chevies is only 2". I suppose you could cut them down, but the way that they're made you'ld probably weaken the shim.
I also question as to why only on one side. Those shims are meant to be used as a pair, and only when the amount of caster change is equal. It's making me think that the axle might be twisted. I'd suggest leaving it out, then have the front end alignment checked at a shop.
