The next size up would be a 185x15. That would also fit on your stock rim. It is the replacement size for the 122s wagon (which came stock with a 6.40-15). I have also read about using a 205/70x15. That would be much less expensive, but would require a 5 inch rim.
Jerry, after checking with the 3 big tire stores around her - Sears, Goodyear, Town Fair Tire - none were able to come up with anything other than the stock size, which I finally went with. I'll keep your post in case there is a next time for tires.
Last week I bought a set of 205/70x15s for my 122 wagon. This was the closest I could find to the original diameter. Actually, they are slightly shorter than the original 6.40x15 tires, but they fit nicely on the stock rims and there are no problems with the body. They actually look like they could have come on the car. Since wagons have the lower rear axle ratio and did not come with an overdrive, the taller tire is needed. I have added an M41 transmission, so the car will go as fast as I have the nerve to drive it (drum brakes all around and no booster).
I have the stock hubcaps on my '64, the ones with the red "V" in the center. Ford wheels will fit the car, but are set up for smaller hubcaps. I have heard that there were Mercury wheels that will work and are set up for the correct diameter hubcaps.Does anyone have any information about that?