I have an oil leak on top of my valve covers at the bolt holes and the filler neck. I modified them to fit my pro-mag roller rockers using golen's website. I do not have any puffing out of the filler neck or dipstick. I have changed the pcv valve. What else could it be?

 

Maybe you could get some sealing washers in a similar size to the aluminum washers the stock bolts use. sealing washers have a rubber seal on the inside portion of them just like the ones that are used for the back of the heads and steam lines or some that have rubber on the bottom of them.