As luck may have it, the drain plug was not missing. Spent Saturday to r&r the tranny. I could tell it had been out before probably a used or reman one, my guess would be the former rather than the latter. Whoever had it out prior must've replaced the seal & didn't seat it in fully. It popped out with 1/2 twist of a screwdriver. Replaced the seals & did the rear main as well. After getting it back together I fired it up & idled for about 10 min & no leakage so far. Will see what happens when I give it a roadtest & if the tranny is any good.