I did what I could and wound up giving up. Measured small, good. Cut too thick a chunk of plywood, requiring support in places I hadn't thought of before. Too many challenges for the benefit. The problem leaves me reverting back to putting dreams of this treadle aside. Thank you to everyone for your support.
One success! The pot metal the hinge pins were set into were garbage. Holding the pins in my hand, looking longingly at the holes for the hinge pins and wishing... Finally, my DH had a grand suggestion. Fencing staples! They hold the pivot pins securely and the head can swing up and back correctly. I did predrill for one side, the other was the hole left by the screws. I will post pics if wanted.