It's only 6800 pounds, what could go wrong?
Keep in mind that the weight of the boat should be resting on the keel, so the pads are only holding the boat upright. You're on the right path, but will be a bit constrained by the cradle frame.
Take two jackstands and an appropriate length of chain. Position one on each side of the boat immediately forward or aft of the cradle pad as appropriate. Put 1/2" plywood under each corner of the stand. The base of the jackstands are triangular; position the stand such that they are angled in toward the hull and the points of each base are pointing toward each other (look around the yard at other boats for an example).
Connect each stand with the chain. THIS IS EXTREMELY IMPORTANT - the stands can be pushed away from the boat if not chained together
. Raise the pads on each jackstand so that the contact the hull. Take up all the slack on the chain, and pull the stands away from the boat to take up any remaining slack. THIS IS IMPORTANT.
Tighten the jackstands as much as you can by hand. They should not be supporting the boat. Now take a deep breath. Loosen ONE
pad on the cradle a little bit, give the boat a good push with both hands to make sure that it doesn't shift. Keep backing off and shoving until the pad is away from the hull. The idea is to make sure that the jackstands are solid and not going to move.
Do the work that you need to under the pad, then restore the cradle pad to the original position. Repeat the procedure for each pad. Alternately, you may be able to take pressure off both forward cradle pads and do your work, but I would restore the cradle pads at night or extended periods.
I've done this many times and it's not that bad. I don't have a cradle, so I'm not as constrained by space. I personally don't like cradles since I can't add pads where I want them.