The reproduction side rails for my late-year 1930 Cabriolet (68B) measure 1 23/32" thick at the front and center section; the rear 25" of the side rails have an extra piece on top of the rails which is the same contour and width of the rest of the frame, so combined, they measure 2 3/32" thick. Also, as I understand it, originally there was at least one layer of 1/8" thick mounting pads between the metal chassis and frame wood, more if needed. I'm grappling with the same issues on my car because the original wood is in pieces. Hope this helps!