1983.. I think it was the year before they hyped about 49% improvement in quality..
79 to 85 were nearly identical under the skin, 'cept for some extra power. mid 85, ford used a throttle body setup on the gt350 if i recall, then went to a SEFI setup in 86. also in 86, they changed the mounting of the front lower control arms. they widened them about 2 inches, giving the car a much improved handling. the sefi 5 liters topped at 225hp i believe. they also moved to a better quality 5 speed in 86. before that the t5's tended to break rather easily. if they didn't, then the 7.5 rear diff did. they were pretty badass till about 92 or 93, when ford made them less powerful, and heavier.
if you manage to find a fox body mustang, or even an sn95(which isn't much more than a modified fox) that hasn't been beaten on, then you've got a "new" classic, or an excellent platform for hotrodding.
i should also add.....when i first saw the pics of the 015, i thought.....blech. it's been growing on me, and i owe much less on my black beauty than i could sell or trade her for........and there will be a test of an 015 in my future.