Something doesn't seem right about that doner description. Vans in '72 were leaf spring, not coil. Also a "3/4 ton" van could still be 1/2 ton components as the vans are rated differently than trucks. If it truly is an HO56/72, then yes the 3rd member will swap, but you need to verify that it is actually the correct diff. I've personally never seen a van with the HO 56/72 axles.
