valve cover gaskets

If the tops of those heads are not machined flat for the gasket, take a good look at the ends of the covers and make sure you don't have a big gap.
A lot of the factory heads are arched up in the center causing fabbed covers to leave the gaps. (press down on the ends of the covers and see if the other end pops up)

Also make sure the covers are not actually just sitting on the intake runners not allowing them to sit on the head itself. (can't tell in the picture)