Yep, 1970-1976 Grand Prix. Looks a whole lot like mine. Someone took off the big chain wheel and the front derailer and possibly replaced the rear. To get a better date you'd need some more specifics on the original components.

Of course you can compare yours to the catalogue:

http://www.sheldonbrown.com/retroral...rand-prix.html

Are you sure the first digit in the serial number isn't a "Z" ?