Thanks all.
Turns out I have access to a 2-56 tap. So I'm going that route. Is it true that the screw mounting the lionel shim to the bottom of the passenger car chassis is 2-56? Anybody know for sure? It's bigger than the ones used to mount the kadee coupler box to the shim. Those are v-e-r-y tiny.
For the record, only one car of the 6 I purchased failed to have the holes tapped. It was the baggage car of the 4-pack. All the others were tapped, and I just finished installing the kadee 747 couplers. Not too painful, and I only lost 1 kadee spring!