Those 4:10s were in the trucks with automatic trans. I found mine via a 'wanted' ad right here on the bolt, best thing I've done to help the old 3/4 ton flatbed of mine.
Those trucks are still on the road all over CA, not as popular as the half ton, usually cheap. You may be able to buy a beater with the rear end you want and swap gears and resell.
Best of luck
