If the 67 Riv is a GS model, it should have a 3:42 posi. If not, it's probably a 3.08. Either way, it looks like you'll have to swap the ring/pinion. Both should be the 9 3/8" rear end. There is a possibility that the LH carrier bearing may be different between the two as there was an early '67 change to increase the size of that bearing. That's why you see carrier bearings listed as '66-'67, and '67-'70.