You're also going to need to change the rear end gears to something numerically lower. Either 4.10s or 3.90s.
I used to run a Crane Mechanical Commander, .648/.669 with the combo you had below, with an 8-71. It ran consistent 9.60s with a mere 12 PSI and 9.5:1 compression in the GMC when it weighed in a 3700 Lbs.
I used to shift it at 6800, but I can't remember what it was going through the traps at.