OK, have you looked at the gearbox? The TH400 has a stumpy extension housing in comparison to a Tri-Matic or TH350.
I have wrecked out many Statesmans over the years and most have had TH400 (Chev Turbo Pattern Blocks). I have 2x 11QT 308 engines here and both are turbo pattern blocks.

Is there a reason you care? Or is it just for the sake of knowing?