With your current setup to switch to a modern rearend, you will need find a donor rear the right width, get a different tranny, (closed tube three speeds can't be converted, 4 speeds can) fab and weld up new spring perches, (make sure to get the pinion angle correct) correct for the spring locating pin being too far forward (there are a couple ways to do this) and have a driveline made to fit. You also will need to figure out how to fab up the parking brake cables depending on which rear end you use.
To some this is not complicated, to others it would be. If this sounds like more you want to get into consider the 3.55 gears. Nothing modern will bolt right in. I believe that switching from the Huck brakes you have now, to Bendix ('51 and later) is a good upgrade. You can do this with your current axle. However, a lot of folks are happy with their Huck's. Good luck,
Steve O
