After almost 45 years, any bolt that is still in it's original hole is something of an anomaly all by itself. I don't know exactly what the reason for the length may be in this instance but, a small block Chevy head has several manifold bolt holes that fall directly above a pushrod. If the bolt that threads into any of those holes is too long, it will contact the pushrod and wreak mayhem when the engine starts. You might try to recall which bolt was in which hole and note the relationship between those holes and the pushrods.
