Oil Leak - Pan or Main

[DesertBob]

My 1985 350M motor has an oil leak. Does not leak when engine stopped. Start engine and it drips. Has coated the bottom of my SM465 transmission so it must be dripping pretty well when driving. When I park the truck I get a silver dollar sized drop and then it stops. No leaks from the intake manifold or oil pressure sender. Would this be a pan gasket or the main seal? I would think a pan gasket would drip all the time but since this is my first sbc chevy I just don't know for sure. If it is the main do you know if the 350M has a one piece or two piece seal?

Thanks!