Which side of the axle is the thick part of the shim on? You'll want it to the rear of the axle to give you the positive caster. I don't think that they make the narrow shims anymore, only the 2 1/2 and 3". I don't have the Northstar catalog handy, it's at the shop, but I can check.
As to a spec, I'd say about 3 1/2 to 4 degrees, anything deeper than that will make the steering effort much higher.
I also would sugest that you not remove the bottom leaf. Maybe the one above it would be ok.
One question, are all 4 tires the same size? Going to a smaller diameter tire shouldn't have changed the caster that much, unless you have a much taller tire on the rear.
I will make one suggestion if you have no luck finding shims; try a shop that does alignments on semi trucks. They should have the proper equipment an possibly the shims needed to get the caster in where it needs to be. I'd also suggest longer U bolts instead of removing leaves.
