Are you going to use your original wheels? If so you need to find a hub that fits your wheels. Get yourself a 1-1/4 inch bearing with the flanges to see if it will fit on your kart. 1-1/4 inch axle will stiffen up the rear, they are all tubular that I know of. The one inch with the threaded ends are solid. The axles with the snap rings take a clamp on type hub, you can set the width of the wheels wherever you want as the hubs can slide in or out, go with the 38 inch axle if you do the snap ring type axle, you can set the wheel thread in or out, the adjustment is infinite. Different axle lengths will not change your wheelbase, I think you are thinking wheel thread, the width of the wheels center to center. You can get a one inch axle with snap rings, then you can use the same size axle bearings if the 1-1/4 inch bearing won't fit your frame.