I would get a kit. I don't think that general washers have the same density as a shim, thus giving way to some compression or bending. If you torque the bolt and later the shim compresses while you are driving the bolt just became less tight. Most people put in a isle of a hardware store for bolts could not find the right size and strength bolt. Also, I believe a shim kit would resist corrosion better. Seeing as someone could be driving on the same road as me someday, I say get a kit and do it the right way the first time.

That all I gotta say on "rigging" sh|t.