The Borg-Warner rear (yours) came in two varieties - High speed (preferred) and Low speed. There may be an aluminum tag attached at the casing halves which can tell you the gear ratio. The tag is described in the Service Bulletins.
I have seen but one low speed version - which is currently on Facebook Marketplace locally. He has $300 for it - not complete (missing the driveshaft, torque tube and radius rods) and its been sitting for a while. I think most B-W axles you see are high speed.
Do you have anything between the BW four speed and the crossmember which the rear axle pushes against? Usually a "connector shaft" of some sort. Possibly a dual high from the earlier AA production/use. Infrequently a Warford three speed - one of which is an overdrive. And I believe Ruckstell made a unit for this use too.
You'll want to take a look at the Ford "Green Book" which describes in some detail the available parts for your truck. An AMAZING amount of later production continues the 1-1/2 ton components. Green books show up on Ebay frequently. Sometimes at considerable price - but shop around. There is a CD available too. Below what is on Amazon. I paid half of that on Ebay.
Good luck! Looks like fun. Just don't be in a hurry!
Joe K