The rear main seal is a very common leak spot on these engines. The originals were made of rope and didn't stand the test of time. Especially if the car sat for years. And although it can, and has been, changed with the engine in the car... I can't imagine it.
I can tell you that my rear main leaks less now that I drive the car a bit more frequently. It seems to have been rejuvenated a bit. I have also switched to a high mileage oil which in theory helps with such things, although I have always been skeptical. Either way it leaks less than it used to.
