don't pay that ticket. not all trucks had shoulder belts, only optional until 79 or 80, i believe. (not 100%). some of the earlier cabs don't even have the mounting holes for the shoulder anyway. if your 77 has the holes, might as well get that fixed. they can't PROVE you took them out, all they can do is tell you to have it fixed. after all, you haven't been the sole owner since 77.
oh, if you do have to go to court, you can defend yourself based on the fact that the police man was not a Chevrolet engineer and thus has no legal technical background to say whether or not the truck was supposed to have the shoulder strap. his opinion is not fact and sure as hell not law. his ticket is therefore not based on law or fact, so it's worthless.
good luck!!!!