Using the weight of the lintel involves either getting it up there and bringing it back down (while we raise the second upright) or else we need guy ropes capable of withstanding the 7 ton strain.

I'm concerned that such a load being taken up by the guy ropes as the levers are operated will produce enough stretch that the levers will never achieve their full pressure on the stone before they run out of movement. We need a method that can absorb the stretch in the rope first and then continue to apply movement to the stone.

I can’t follow the current discussion very well, but can you explain why the 100 ft pole idea isn’t being pursued, as it seems to involve the least effort of all in my garden.

Although, for a stone lying at a shallow angle, the pole needs to stick very high in the air, it would seem we could achieve this fairly easily:
Raise the pole Horizontally on towers under each end. Build a third “pivot” tower underneath it, just right of centre. Remove the Left tower, and allow the pole to pivot downwards until it rests at right angles against the base of the stone. Attach a rope to complete the triangle. Remove the remaining towers.
This seems to involve little or no stone-raising towers, just pole-raising towers so has got to be worth thinking about.