Two or three positives to the front batt back to the back batt bank and two or three runs of negative to the front batt, back to the batt bank, will be more then fine. I would take the headliner down at the B pillar forward and do a layer of 3/4 birch, and then maybe throw some fabric over that, should help with the flexing big time. I would go with Second Skin Spectrum Sludge for the doors and the floor. You will not need alot due to only having to deaden from the B pillar to the A pillar.....
Good to see the customer got his mind right and decided to wall it, should be nasty.....