[slotard]

Seized brakes (or similar) wasn't something I thought was very likely, just a possibility.

You can tell tranny by pan shape. I know you said 4 speed but TH350 has a rectangular pan with a corner cut off, TH400 is oddly shaped, 700R4/4L60E is rectangular, 4L80E has one square end and then uneven corners removed on the other end. If you search "TH400 pan" on google image search you can get images.