Adding the cup washer may change the axle caster, placed on top increases it a little but should hopefully still be within limits. Some sources say 3- 6 degrees, while others like 4-5 deg. as an average. Also, you may need to tack the vendor's washer if needed to the top clamp or some tack it to the ball to keep it from migrating. The stock style clamping parts are supposed to have a small gap between them when the springs and cotters are seated.
If memory serves, the kit should come with new correct length bolts.