I actualy have a diff with the early 403 ratios but with fine spline axles. Apparently there are a few around. It was a pain when I got it to replace my blown diff because I had to get new axles too.Trevor , as Graham says - late 403 diff has fine spline axles ( better but not absolutely essential ) and has different diff ratio , so that if you go that way - change to a later gear box - if that is possible!( Graham will have a better idea as to later gear box into 1951 203 - different gear box mounting!) You need to check whether the 403 diff you have is fine spline or early - if an early - diff ratio is ok but no fine spline. Then you will need axle tubes from a late 203 - they bolt onto the 403 diff centre. You can fit fine spline axles to the early 403 diff if you change the drive gear in the diff centre - last if using the fine spline axle in the later 203 axle tube - you need to change the seal in the outer part of the axle tube about 6 inches in from the end - a fairly easy drive in/out with a suitable length of pipe.