I've got the same leak on my new engine, I'm assuming it is the oil pan seal to the rear, a simple fix, but I'll use way more gasket seal next time around. I will agree with some of your responders that my first go around with this issue was a result of a leaking valve cover gasket, and may be the problem now as well. What I have noticed which leads me to believe it is the rear pan seal is that there is no leak when it is on all fours, when the front is jacked up in the air is when it leaks. Like you I hate leaks, and on a new and very clean and meticulous restoration it drives me nuts. I will say this every engine I've had in this car, three, they all have had this issue. Apparently I don't get it when it comes to pan gasket installation, I may hire the next one done so I have someone to bitch at. Good luck! And let us know what you find.