I just made seat risers for the Olds 60/40 power split bench seat in my '53 Chevy pickup last weekend. I made a rectangular frame of 1 1/4" square tubing that the seats set on. It is sized so the frame rails align with the mounting holes in the bottom of the seat. Then I cut some legs that weld at the corners of the rectangular frame that space the seat height properly. Tabs were welded to the feet to bolt it to the floor. The seats bolt to the frame with holes drilled through the square tubing. I will drill some 1/4" holes in the tubing to attach upholstered trim panels. Requires a little welding but results in a perfectly fitting, very safe solution.