1951 3600 is a 3/4 ton with 8 lugs. If you are trying to replace the 3/4 ton axles with 1/2 ton 6 lug axles, look for a 1960s 12 bolt truck axle, or a 10 bolt truck axle from a 4x4 up to about 1988. Those axles are relatively common, have a ton of gear choices, are usually pretty narrow between 61 and 64 inches wheel mount to wheel mount, depending on year and application.
Any suburban axle after 1990 will likely be too wide... unless you like the look of having the wheels outside the fenders.
