I'll echo some other folks. Make sure the surfaces at the heads and manifolds are spotless and bolt them together, watch the torque value also.

If you are actually using headers from hooker or dougs etc etc etc... then the flange that goes against the head needs to be as straight and flat as possible, You can use a 4 or 6 foot level as a straight edge and if you can see daylight between the 2, you'll need to lower the highspots. The collectors - I have used the aluminum gasket from Earls ( I think it was) and it worked like a charm.