Dear Mr. Proctor,
If I remember right, the old and new Mantua 50" drivers are slightly different in diameter and have different flanges, but have the same axles. What kind of motor does this Shifter have? If it has a PM-1 with a nylon worm and nylon gear, the gear should also be the same for both kinds of drivers, so you should be able to buy two of the newer style and replace both sets. This costs more than one set, of course, but you do get smaller flanges. You might also press the tire off the driver with bad insulation and replace it with a piece of index card or fiber gasket material. The fiber material is available at your friendly neighborhood auto parts store in several thicknesses. Just cut a strip with the grain, wrap it around the center, and press the tire back on.
Cordially yours, Gerard P.