Jim: The additional caster shim(s) should help a lot. You should get the front end aligned after installing the shims, and tell the tech you want between 5º and 7º of negative caster. Adding caster shims will make the steering heavier, especially at low speed in parking lots, but it should feel MUCH more stable at higher highway speeds.
Of course, adding balanced weight to the front is always a good thing. Finding a place to put it can be a problem. Porsche's weights low at the corners in front of some 911s is just about perfect placement. We would have to do some serious fabricating of a structure to hold them on a fiberglass 356.