Majority are uniflange. However I owned one that was a BOP only flange. Wouldn't have believed it if I didn't see with my own eyes. No tag on it but I always suspected it was an '81' case.
So depending on the history of the vehicle, it is not a certainty that it will have a uniflange pattern given the propensity for "corporate" transmission shops to play musical chairs with boxes during their "standard rebuilds".
Trust but verify.