I put a power seat base in my 67 from a 69 Gran Prix. I had to order that brace your talking about. My seat base fit the front two attachment points, came up short for the inside rear, and the outside rear was hanging up in the air around 3" if I remember correctly. You might try bolting yours in on the front to see how tall you need to make the bracket your going to fabricate for the outside rear. I made a strap to grab the inside rear mount and attach to the original floor mount. Did this 8 years ago and didn't take any pictures or measurements though.
