I used a building laser to set the heights but a level works too; just find the spot that's the highest or where you want the floor to be and start there.
The supports are just 3/16 flat bar bent into a "C" with "feet" which are drilled and rivetted to the floor pan
They were roughly laid out and riveted down; the ply positioned and laid on top of them and holes drilled through both; got a few extra holes in the ply
10-24 rivnut into the FB, countersink the flat head screws and done
At first I tried to do a one piece but I couldn't get all the edges to fit tight; the template tool has to stay at right angles to the centerline of the car.....sorta
I overlapped the ply a bit in the middle; snapped a chalk line down the center and drilled a 1/8th hole on the line at each end so the bottom piece was marked as well.
I used a butchered pick-up canopy latch minus the handle for the spare tire lid...
I think I ended up with a drum effect too; car seemed louder