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[email protected] 2x" width="20" height="20" /> well you've managed to run into one of the oldest performance related problems the car/truck enthusiasts have ever faced. Header leaks have been around since the header was invented. There are a number of solutions, some will help you some won't.
To start with always try to buy good quality headers that use thick flanges. Cheapie headers have cheapie flanges and these will invariably warp & twist with heat cycling and eventually leak. Not a lot you can do about those. You can/should use premium header gaskets. Some have a small bead of hi-temp silicone around each outlet to help seal it if the ****** shifts or warps. This should be the first thing you try. You also need to frequently re-touque the header bolts. Just installing them & torquing them down isn't enough. They'll loosen up with heat cycles and then leak so you have to keep on them. If you hate re-torquing the bolts every month or two then you can buy special header bolts with locking tabs that prevent the bolts from backing out & are guaranteed to stay torqued. If the headers are old/worn or just been on there awhile you can have the flanges reground (assuming they're thick enuf) so that you have a flat surface & can start again. If they're NOT thick enuf then you can have them welded to add beads of material around the exhaust ports and THEN have them ground and use new, improved & expensive header gaskets.
Stock manifolds seal well because there's a lot of material there to both clamp down on the gasket AND keep the manifold from warping. Headers don't have that material so they have a tendency to warp after numerous heat cycles. Not a lot you can do about it, it's the nature of the beast.
Get yourself a new set of permium header gaskets, use good bolts & retorque frequently. If that doesn't work get another set of premium gaskets & a set of locked header bolts & be done with it. If THAT doesn't work then you're probably looking at having the ****** reqround. Not necessarily cheap or easy but that's pretty much what needs to be done.
