The Stovebolt Site Forums The Shop Area Driveline '51 3600 3/4 ton rearend swap

Any overdrive conversion be it a T-5 or an SM319 will require a driveshaft modification for that year truck. Since it has a "partial" torque tube it will need to be converted to a full open drive shaft. As 66submarine stated there is a 410:1 rear end available (from a 67-72 truck) but that's IT. As you mentioned if everything else is worn out it may be better to replace the entire axle housing assy. Someone else may know what later model housings may fit. Any thing you use will likely require spring pad relocation & welding.
Fred