transmission identification

I've got what I'm assuming is a 904 transmission behind a rebuilt reman 318 engine in my 68 Barracuda - definitely not original to the car. The transmission is slipping some and when I dropped the pan, lots of shiny gunk and metal shavings. I'm planning on just replacing the filter and pan gasket for now, but I suspect in the near future I'm probably going to have to rebuild the thing, or swap it with something else. I'm trying to identify just what I have - I found the list on mymopar and narrowed it down to an 81-85 904 or 999. The stamped number above the pan on the rail is:

PK4058398 8390 7713

Looking at some of the other casting numbers, it does look like its an 83. Does anyone have any other reference manuals or any way of helping me narrow down what it came out of a little farther?

Thanks!
Chris