I jacked up the plane using a little bottle jack, c-clamps and oak 2x2s along with engine jack on the front.
I used a simple laser level mounted on tripod at tail. It would draw a line on the floor that went up the center of the nose wheel leg and along the bottom of the fuselage. When adjusted, it would split the center rivets on the bottom of the fuse.
What's not shown here was a 6' - 8' length of 1/2" angle iron. I laid that on the floor to mark the center line. Then measurements for the front and the rear of the main wheel pants could be easily be taked from the angle iron. You need to use something because the laser is only visible if the lights are off. The weight of the angle iron (not aluminum) had enough weight to not be moved easily during measurements.
I came away from this very confident about the alignment of the pants. AS MANY HAVE STATED, DO THIS BEFORE MOUNTING WINGS. In any case, a good pair of knee pads help.