another option is to get ahold of some high quality sand paper (something like 80 grit), and a dead flat surface. set the sand paper on the flat surface, lay the manifold on the sand paper, and sand away. you'll know all the parts are in one plane again when the entire manifold surface has a nice shiny new surface. this will only work if the manifold is only slightly warped. probably not the perfect solution, but it's cheap and should work. I did it on a nissan manifold once, but never ended up bolting it on the engine.
I just took mine off and replaced the blown gasket with two new ones. Fixed the leak and its been 6 months or so. If you have a none blown new gasket and the manifold is tight and its still leaking then this may not work. A little copper RTV never hurts either.