Even on clean original unmolested cars engine can be sitting in there wrong due to extremely sloppy factory tolerances. Now add all those other factors age, ppl swapped k members, prior damage, aftermarket motor mounts, mods etc then probability of engine not located correctly goes up.
The header manufacturers have to assume a certain engine placement. My headers just barely clear lots of things. Engine placement off just a little they would be hitting hitting torsion bars, steering linkage, frame rails, steering shaft ect.