I am assuming you have a 351 W with a 2 piece seal. When I got my block from Keith Craft, brand new, it leaked the first time I fired it up.
I got in there once and replaced it and it still leaked.
I got in there again and replaced it, this time I clocked the seal just a tad so that the ends of the seal do not meet up with the edges of the main cap and block. You don't have to go crazy with the offset, just about 3/8" or so will be fine.
I also used a Napa seal 2nd time around, and I used the slightest dab of good gasket sealer at the outer edge of the cap where it meets the block. So far I don't have a leak.