I found your proposed rear brake upgrade using the 65 V6 Mustang hub interesting, so I spent some time researching if it was possible and here’s what I found.
The Capri’s inner bearing will work, but the Mustang’s outer bearing for the 4 lug hub doesn’t look like it’ll fit the Capri spindle. The Mustang’s outer bearing has an inside diameter of 0.687” while the Capri’s outer bearing has an inside diameter of 0.75”.
I think the confusion comes from the fact the V6 Mustang had 3 different brake options, standard drums, heavy duty drums, and disc brakes. It’s the 5 lug HD drum hub that has the outer bearing that will fit the Capri spindle, unfortunately it’s 5 lug and the inner bearing is too big in diameter and width for the Capri’s spindle.
I also did a search thru some bearing catalogs and found the NTN A6075/A6157 bearing that would fit the Capri’s outside spindle, but it would still require 45 thousands to be machined out of the 4 lug V6 hub. It would also require spacers due to it being narrower in width. The other issue was it only had static/dynamic load capacity of 12,800N verse the stock Capri’s 29,000N. So how long it would last would be highly questionable.