Generally speaking at 0 deck flat top piston with a 79.5 cc chamber is right at 11:1, depending on head gasket. That said, to be a 12.6 with milled 906's...is a very generous GUESS-timate.
Track altitude is 700ft, my shop where I measured cyl pressure is 600ft.
Have no opinion on comp ratio, just referring to Hughes etimation...
70 W100 Power Wagon. 318 4-spd 70 Sport Fury 440 2dr HT 71 Duster 340 71 Charger Super Bee - 383/727 72 Charger "Sabotage" - 440/727 - Street/Strip 78 Warlock in beautiful patina