When installing the headers to help eliminate leaks run the truck for a bit (a nice drive with the new headers will be fun anyway) let the engine cool down and retighen. Repeat this several times generaly 3 to 4 times will do the trick. The reason they leak is that the gasket hasn't seated yet and while its going through this process the bolts will losen up and allow leaks.

 

I have a set of the Summit Headers, made by PaceSetter. I didn't have any problems with them sealing. They have the thick 3/8 inch flange. I had to cut the stud on the back side of the twin port, on both sides, I made sure that I left them long enough so that I could still put the stock manifolds back on if needed. Only needed to take about 1/2 inch off. I used a dremel tool to cut the stud. I did not use the gaskets that came with the headers, I bought a set of Felpro gaskets from autozone. Didn't have much problems getting at all the bolts. Some were tougher than others, but that is kind of expected with headers.

I did what IndyRamDriver stated as well, be sure to retighten a couple of times the first few weeks.