I believe the part number 3879208 is for a flange that takes the U bolt clamps to hold the U joint in place which would be wrong for your car.
If your rear is original to your car, it would take flange 3954393 which was later superceded to 3996118. Those flanges have straps that are bolted in place to hold the U joint.
Here is the flange a '70 Nova 12 bolt would take on eBay...
https://www.ebay.com/itm/Rare-Origin...NbU2jy&vxp=mtr
Here is an auction showing the straps and the U bolt differences.....
https://www.ebay.com/itm/DRIVESHAFT-...!US!-1&vxp=mtr
Hope this info helps you out......Warren