I believe the standard auto transmission on the '78 was the Turbohydramatic 400 (THM400), which is a very well known box, used on Commodores of the period (VB, VC, etc).
According to my documentation, if it's a THM400 there should be a Build Code on it such as: 78K or 78Z and underneath that there should be a model and production sequence number such as "78CAxxxx", where xxxx is the sequence number.
PS: This thread should be under Transmissions - Differentials, not Forum News.
moved. Admin