Oil Leak: Oil pan gasket or rear seal?

Bought a black light/ dye gizmo to find out where the leak is on a 289 with manual trans. The oil appears just behind the back of the oil pan, at the low point where
the rear seal cutout is in the pan. Also, the oil seems to only come out when driving, when the oil is sloshed around as best as we can tell. Can easily lose a quart in a few miles. No oil pan damage, bolts tight. Any guess weather rear seal or oil pan gasket? Any advantage to replacing rear seal as long as the oil pan is off? Thanks, Dave
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