Here’s a way they could have put the big pieces on top of Stonehenge.
Let the giant rock lay on the ground.
Dig a hole in the ground under each end.
Build a giant wheel around each end so the rock goes through the wheel about halfway between edge and center. The distance from the top of rock, in this position, to the top of wheel should be greater than the end height of the bottom of the rock above the ground when it’s in place.
Put an axle through the center of the wheels. If the wheel is a clock, the axle is where the hands attach and the rock is at 6 o’clock, put in poles between the wheels at 9, 12, and 3.
Attach to the extra poles a trough that can rotate around the pole.
Fill all the troughs with rocks and water until you get even mass all the way around the axle. As the wheels rotate the troughs rotate to stay upright and don’t spill.
Roll the rock into place. In fact, it might be easier to place the top piece first, then set the bottom pieces and dismantle the apparatus later!!!!!!!!!!!
With the right kind of launch pad they could have locked up two pieces (positions 3 and 9) or even three (positions 3, 6, and 9), but getting them unloaded would have been difficult.