In 2000 I rebuilt a 409-409. Oil dripping from dust pan from the beginning. Probably under the car a dozen times in the first year trying to figure it out. Found the 4 speed leaking out the front and oil pressure switch leaking. Fixed them but still dripped but not a significant amount. The engine now has 39000 miles on it and the drip is still the same. Just put cardboard on the garage floor and go the the car wash about 3 times a year to wash out under the car. Just doesn't seem worth it to pull the engine to replace the rear main seal and still possibly get the same result.