Same car here really.
'73/360/4spd-4.10's
The bottom end improved and the top end screamed. Then came the move from a Torker II to the RPM/AG. It got even better. Much more bottom end and about the same top end. Slightly better in its feel. The car ran smoother through the range.
I am running the 60779, regular heads/spring set up. The increased spring rate on the "775" head should be just fine, just check the head out and make sure the springs fall into the proper rates for your cam.
