Bolts that are as long as a rear axle tend to only have their threads for a couple of inches, not over the full length. Coaster axles tend to have rather fine threads for the diameter, 38"x24tpi being a very common spec for older American made coaster hubs. A quick search of two industrial suppliers show, for the aprox overall length needed, only about 1" of threading. Much better would be a Wheels brand axle in Cro Moly. Or if the remaining threads are not too far gone a thread chaser repair. Older LBSs might have the Bicycle Research thread chaser kit which included both a 3/8"x24tpi and 26tpi chasers.

If you are near me I can help with that. Andy