I would think the best approach would be to swap front hubs to the single wheel type. Then the backspace dimension needed would probably be the same as the 19.5 inch wheels you're likely to find.
My '55 1st GMC single wheel 1-ton had a set of 19.5 wheels on it when I bought it. They fit fine. I no longer have those wheels.
If you have dually type hubs up front any single wheel type wheel is going to mount far outside the fender.
