What trans do I have?

The slip yoke tailshaft was introduced in 1966. The B/C-body versions used the 30 spline output shaft, which is the same as the 727. The A-body versions used 26 spline output shaft, which is the same as the 904. In 1967, 383 Darts and Barracudas ordered with a four speed got the 30 spline output shaft. All other four speed A-bodies kept the 26 spline version for '67. In 1968, all A/B/C-body four speeds got the 30 spline output shaft.

I'm guessing the 1967 383 Darts and 'Cudas got 23 spline input shaft versions of the 833, although I'm not completely sure. The only other way to get an 18 spline input shaft in an A-body would be to get a 1968 Dart or Barracuda with a Hemi/four speed combo. B/E-bodies could have either an 18 or 23 spline inputs, depending on the engine.