The center jacks will not hold up the front, as the system is designed to transfer fluid between the center and rear jack on the same side for front to back leveling. The front jacks will retract all the way and still be touching the ground, the center jacks retract and the fluid moves to the rear jacks. I've actually lowered the front completely in my driveway, while trying to diagnose my Auto Leveling system, and the front jacks will never leave the ground unless the truck is connected, or you have the frame blocked up somehow.
Now, for your issue, you could carry some 6 ton Jack stands and set them up under the front part of the frame to support it as you retract the front jacks to put blocks under it.