I installed 3 point belts into my 1953 panel. I happened to have some 4"x4" x 3/16th plates with half inch holes in the center. I taped it to the end of a wooden yardstick and positioned it behind the sheet metal (there is a bit of room) They have been there for many years, never tested in a crash. The belts came from early 90s Chevy pickup.
