Did you make sure the header flanges are nice and flat? Are you using header gaskets?
With my junk I take the headers to the machine shop and have the flange surface put down on their table-type sander to get them flat. I then use the red copper RTV instead of a traditional header gasket.
If the header to head surface has any warp to them, the chance of a leak and loose header bolts multiply.
__________________
General Stonewall Jackson has been quoted saying that when the time of war comes,
“My advice is to draw the sword and throw away the scabbard".