[ford38v8]

That's not your oil pan, it's your clutch housing. The cotter pin is there to prevent that hole from clogging up. Oil is expected to exit there to keep it off the clutch. Your "mechanic" has apparently overfilled the crankcase, thus allowing the excess oil to dump out the rear seal to the clutch housing. You need a new mechanic or preferably, do it yourself so you know it's done right.