"..A little known fact about factory heads is that the valve cover bolt holes are not square to the rail. They are all tipped toward the intake side. Also the rail area where the bolts are drilled are also tipped. This means a flat surfaced valve cover will not sit evenly on a factory cast head." quoted from TRENDZ

I discovered that when I put studs in the "452" heads I used on my '64 Polara and tried fitting the Mopar Performance cast valve covers on the engine in the car. I had to slot the holes to make them fit