I believe the GTS ended in 69, and that there were just small blocks in the Dart from 70 to 76. Also, GT doesn't mean big block or small block, often GT's had /6.
GT was just a trim level package, 67 68 69, I think that the GTS was the high perf version of the GT, so the GTS was a 340 or 383 V8, GSS was a 440 Dart.