Just so you can get an idea. I sold this one for around $120 shipped I think and it was used.
http://community.ratsun.net/topic/13876-510-wagon-roof-rack-and-hatch-wing/page__p__185799__fromsearch__1entry185799
Welcome.
Looks like a good start. I've noticed that most trucks(ute's?)in Australia have flat beds but yours has an actual bed(tub?). Rusty floor but weld in some sheet metal and your good to go. I have an extra 320 bed if you ever happen to fly your personal jet to California.
Have you guys tried cleaning the little posts where the wiring for the sending unit connects to?
Mine (521) didn't work either but after a good cleaning with
A wire brush ,my gauge came back to life.