A Beck can't use caster shims as the beam is welded in as the frontmost frame crossmember. The only way to adjust caster, short of cutting and welding, is to lower the rear of the car relative to the front. Luckily they include adjustable spring plates, making that an easy job. I had a similar caster issue when I first got mine, but it's easy to correct.
Jim, if you bought your car new you should have an owners manual detailing how to adjust the rear ride height. You can also adjust the front as the car has avus adjusters on the beam. Check with Carey if you don't have a manual, or PM me and I'll send you the instructions. Also, your car should have a full-sized spare up front.