I am wondering...how do I go about changing out my rear main seal?

I've had an oil leak since not long after the motor was rebuilt, leaking in the inspection plate area of the bell housing. A while back I took the pan off...the oil pan was old and crappy in general but it had a gash in the rise where it meets the rear pan seal so I figured for sure that was the problem.

Guess not because I have a leak still.

I believe you can change the rear main when the pan is off without pulling the motor but how exactly do I go about doing so?