Did you ever find those retainer clips for the ball studs on the rear of the seats? I was digging through the '66 today and noticed they are actually in that truck.. I don't have the ball studs though.
No....never got the retainer clips - I saw in the FSM that they look like small square saddles with a hole approx 3/8" or so.
If you get stuck for a rails - I have this pair that I will never use. I looked at the studs and thought they must be for a different model year. I wouldn't be allowed to use them here anyway according to our 'Low Volume Build Vehicle' rules. Seats need to be fixed - and the R clips only just qualify if on the rear.