There were multiple distributors made for the Slant-6, and some of them have different body diameters (and so take different-diameter distributor caps) but all Slant-6 blocks have the same size distributor hole, and all Slant-6 distributors have the same shank size—whether made by Chrysler, Prestolite (Canadian OEM), Bosch (Australian OEM), Mallory, or whoever else.
The distributor should not wobble, and it's not being caused by failure to make up a special spacer dingus that shouldn't be there. If the OP's distributor doesn't have the
correct O-rings, that could be causing the issue; O-ring info is
here.
If new correct O-rings don't fix it, then there's probably something the matter with the distributor housing itself. Could have a chunk broken out of it, could have been brutalized during a "remanufacturing" process, could be cursed by an Egyptian mummy thousands of years ago, or could be a poor-quality Chinese knockoff item.
If this were my car I'd take the situation as an opportunity to do the
HEI upgrade , which involves a new distributor.