But does the OP's bike ("1974 tinker-toy Alan") have an oversized top
tube? If he had standard housing clips previously, then he probably has
a 1" top tube. They don't list the clamps as coming in 1" size.
Shimming it would be ugly, and would add one more place for it to slip.
I'd go for replacing the regular clips and sticking with housing all the
way back, the way God intended.