The rear seat is different; later model ones have the flip-down and hook in the front, snap in the back. The Burb 3rd seats are like that. Early model rear seats just bolt down on the corners.
So, umm, the rear seat from an 89/90 won't drop onto a 73-5, without tweaking anyway. Now, if the 73/5 has the seat hardware, you might be able to mix-n-match, otherwise, I see some welding in your future.
-- A