My 84 GT (vin verified) has a build date on the door tag of June 1984. Very late in the model year. Last 6 Vin digits are 194410 so that should be the 94,041st mustang off the Dearborn assembly line if I'm decoding correctly. Deduct 100,010 from the number.
Anyway, aside from all that, not long after I bought the 84GT a couple years ago I was chasing a perceived irritating lifter tick at certain rpms. So I ordered a set of Ford Performance lifters from Summit and to my surprise after lifting...

Late build date 84 5.0 roller cam/lifters