Hello. I know it's 7 years later, but I had the same leak and I finally fixed it. I had a leak that seemed to be coming from the crankshaft itself. It would drip onto the pulley, and as it spun, it would spray it everywhere. I had to refill a quart of oil once a month. Replaced the timing cover gasket and seal, lower intake manifold gasket, valve cover gaskets, and the oil pressure switch, all to no avail. Recently, I came across this video on YouTube,
https://youtu.be/c1iiGkj1c8w
that talks about the drain hole located on the cavity for the timing cover seal.
Apparently, it's too small to allow the oil to drain back into the oil pan fast enough before some spills out through the crankshaft. They basically said to drill that hole bigger, so I used an 11/64 drill bit. You shouldn't go any bigger than that. In the video, they left the timing cover on the car, but I took it off because I didn't want to risk any metal shavings falling into the oil pan.
Also while I had the timing cover off, I replaced the timing cover seal with the OEM part GM 12582313. When I replaced it the first time, I used the Felpro seal, which is quite different from the OEM. I'm not 100% sure which of the two made the biggest difference, or if they both did, but for sure it fixed my oil leak.