As the others have noted, the Eaton parts will swap no problem. Before you make a 4 hour trip, though, see if the seller can give you the ID code off the center cection. That will tell you if it's a 4.10 or not. The codes for a 4.10 HO72 rear for 1970 are as follows, TIE, TIH, TJL, TLX, TLZ, TMR, TNZ, TRN. There is also a possibility that it might be a Spicer 70, which was also common in the 1 tons of that era, though that is more of a GMC application. Even if it is a Spicer, you could swap the entire rear if you wanted to get the gearing.
