Mopar Performance cast covers are too wide to seal with the Moroso rubber gaskets. The Mopar valve covers are built wrong so you have to use something like the FelPro 1612 gasket to make up the difference.

I posted pictures on here a while back showing what the problem was with the MP valve covers. You can double check by measuring the width of the flange on the covers and comparing it to the width of the head. You'll see that the valve covers are too wide which is why they don't seal. They just twist the gasket and create a leak.