I'd expect he can get $12-13K for :
$6000-$9000 nice '73 4-speed Grand Am
$3000-$4000 non-original 455SD engine
$1000+ Functional Ram Air system
Hope you can get the car. I'd hate to see it go to a non-Pontiac person and disappear. I could only afford it if I traded my car.
For $20K I'd get a restored first gen 400 'bird convertible.
http://fiedlerh.home.att.net/