Your worried about the wrong thing. Typically torque values on hardware are designed to generate a tensile load in the bolt of 75% of the proof load of that bolt's material, but this is obviously based on a nominal friction coefficient of the joints components (bolts, nuts, washers, etc.). So, for example, a 3/8" UNC Grade 8 bolt torqued to 33 ft-lbs on the nut side would nominally generate about 7,000 lbs of tensile (clamp) load. I.E, that is 7,000 lbs of force trying to pull the stud out of the cylinder head, much, much more than the weight of the headers which is distributed across 12 or 16 fasteners.
The much bigger concern is a fastener / nut combo that does not have the correct friction coefficient. Too little friction leads to very high clamp loads that can damage the joint by stripping threads, yielding or pulling the bolt apart, or crushing parts of the joint (gaskets, bushings, etc.).