I've had them leak when the gasket got older and hard. Also, it's possible to mis-align the gasket fore-aft in the rear cut-out area. And then, it's also possible to have a leak at the infamous front cover that gets on the ridge below the exhaust ports and runs all the way back to the bell housing, making you think it's the valve cover.
The bolts on mine always turn down quite easily, and then quickly get firm, like they bottomed out in the head. I assume they are indeed bottoming out rather than oversqueezing the gasket. kinda like the metal standoffs on the tranny pan.

I wonder how you get that crap off the valve cover - It looks about a quarter inch thick. I have been threatening to redo my cover for 15 years.
Let us know how you come out.

DG