The third member is the center section of your rearend, sometimes called the pumpkin that has the ring and pinion gears. It unbolts from the housing and can be interchanged with any other pumpkin of the same rearend (Eaton 3/4 and 1 ton) from about '46-'72. (you have to remove the axles first) The desirable 4.10 gearset that Bill mentions is only found in the later years.
The axle Patrick told you to look for is a Dana 60, also very common in Dodge vans with that ratio. You will need to relocate the spring perches (cutting/welding) to bolt it into your vehicle.
