1971 is when there were a number of changes done, both to the 727 and the 904 in the range of PK35XXXXX and up. Cases, fluid passages, etc. should be the same except for some servo sizes on the small transmission, except for the mid 60s. style units. Internal parts have been changed. I would think anything in the mid 70s would be fine. I'm sure others can be more specific and will chime in.

If that thing was broken that bad, I would check shafts for being bent and closely inspect everything else.