I had the brilliant idea of removing some original stock wire looms to have them chromed. The bolts attaching the looms to the intake manifold also happened to hold down the manifold, too. Never seen so much steam and oil as the first time I drove it like that. I thought it was a blown head gasket or cracked head, too, until I realized what I'd done.
Anyway, I pulled the intake manifold and valve covers and saw the same goop in the lifter valley and valve covers, too. The goop was only toward the back of the valley, below the spot where I had pulled the bolts.
I sprayed a little engine degreaser in the valley and the top of the heads, scrubbed the tight places with a toothbrush and wiped it up with some paper towels. I installed a new intake manifold a couple of days ago, so next step is an engine flush using clean oil and filter to catch any junk, then another oil change and fresh filter to drive with.
If you opt to pull the heads, I learned that "magna fluxing" (help me out guys if I'm getting the name wrong) is a good way to check for cracks in heads. The shop dusts a yellow magnetic powder on the heads, then sends a magnetic field through the heads that attracts the yellow powder into any existing cracks for detection.
Below are "goop" and "no more goop" pictures for anyone's reference out there. I'm following your saga and hope you find the real source of your leak. I'll post anything good I learn in case it helps. Good luck!