Best way to check a valve seal is to put new plugs in and run them for about 100 miles then pull them and check for oil. It also could be cracked heads. I have seen a few heads crack near the bolt holes and cause oil to leak. Its not uncommon on the 5.3L heads, I can't remember the casting numbers off the top of my head. I want to say 862...

Looks like you have other issues though, did you at least use some loctite on the head bolts and do the correct torque sequence?