Rear main oil leak when parked on a steep hill.

I went to move my 1969 Dart Race Car today and I noticed what looked like a pretty severe rear oil leak. I have a very steep driveway that the car was parked on front end high. It didn’t leak when I first started it up but when I moved it up into my level street it started running out of the bell housing with a trail that started as soon as the car crested the top of my driveway. After it sat for a half hour or so it. Stopped dripping and I fired the car up and it didn’t leak. On a similar note I had what I thought was a rear main leak on my 68 Plymouth with a new 383. It was also parked in the same driveway. I am wondering if the rear main seal just isn’t meant to hold back oil with no movement for extended periods of time. I would nose the cars downward but I have had issues with fuel draining from the tank through the carburetor and into the cylinders. Has anyone else had something similar.