My memory does not work like it used to so I have forgotten many of the finer details about castings. That said there are many resources both in book form and online with everything you would ever want to know, the only thing is to make sure and cross reference because sometimes there are mistakes. Here's one source http://www.wallaceracing.com/head1.htm and scroll down near the bottom part of the page. Being an august date they are most likely a 70 head which Wallace shows' as all having the same size valves. One of the easiest ways to see if a Pontiac head is a stouter performance version is to pull the valve cover and see if it has the heavier duty screw in rocker studs and pushrod guide plates verses press in rocker studs. Unfortunately one of the more pervasive lies some people tell when selling a first gen bird with a non original 400 is to say it came out of a ''GTO'' because it makes it sound better than to say it came out of a 4 door Bonneville. The reality is in most cases if the motor is completely stock the only major difference is the heads cam and timing curve all most likely changed anyways.