Did this leak start after a fresh rebuild of the engine? The reason I ask is I have seen several B/RB engines where the timing cover does not lay flat up against the front of the block. After the timing cover has been bolted on and torqued to spec, you could still run a thin feelers gauge in between the cover and block. This was in the area about 3-4 inches up from where the timing cover meets the oil pan on both sides. The oil seeps out and runs down to the front corners of the pan then follows the rails back. Just something I thought you might want to check before dropping the pan again.

Good luck.

Dan