Looks OK. I think your idea of threading one long bolt through would be the best bet. It would be like a double nut. I think 2 short bolts might work loose as the axle flexes or vibrates. If you don't thread it through the extension nut it might work loose as well. Let us know after the season how it works. I'm doing the bearings on the front axle now but I'm thinking I might have to do the center and back axles later. Good luck