There are two TH-375s: the TH-375 or TH-375A, and the TH-375B. The TH-375A is the more common, medium duty version of the TH-400. The TH-375B is a less common, heavy duty version of the TH-350. Used in certain full size Buicks in the mid 70's.
A longshaft TH-375A is like a longshaft TH-400, but it uses the smaller diameter yoke from the TH-350. Pan shape is that of the TH-400. The only difference is the output shaft and the tailshaft housing. The TH-375A has the guts of a TH-400 with the output shaft of a TH-350. It was used in full size cars, and it effectively shortened the drive shaft. The TH-375A is also often swapped into big cars as a direct replacement for dead longshaft TH-350s.
On the underside of the trans, right where the tail housing bolts up, the letters "TH-375" can usually be found cast into the trans housing.
The TH-375B pan is the same shape and size as the TH-350.