Mustang codes on clic cars>>>code a,b, j, etc.?

im looking to buy a 67-71 mustang gt and i keep finding different codes for the model. i found code a, code b, code j, etc. what do they mean and which one is the most valueable? also, im torn between a 71 fastback and a 70 mach 1. whats everyones opinion on the two and whih ones is "best", for lack of a better word.