If it is a 72-400, its a good block(thicker mains then later years). Not sure about the 455. When did Pontiac blocks go thin(74-75-76)?
Wasn't necessarily "thin"..
Was more of a nickle content and hardness issue.
Pontiac went to a thicker main web block around 70(?) when they stopped using hard steel(Nickle). Then around 74-5(?) they used even less steel. East to check. If the engine # ends in 557, its a thin block. Another way to tell is if the main cap bolt holes go all the way thru. I would'nt use one over 350 HP without a good prep. Over 400 is a gamble. Pontiac went back the 488 casting in 78 due wipe'n out rod-n-main brgs in FB/TA's under warrenty. What were those...180-220 net HP?
