Like LaTech said, they made more than one chamber size of 4X heads. If you have 113 cc's i would toss them, or your idea of making 450hp. Could still maybe reach that mark, but not w/o alot of work that i don't think is worth it.
i did exactly like what you said on my 061 heads for my 400. i think there is a pic of them somewhere in my album. i couldn't possibly have removed more than 2 or 3 cc's in my uneducated opinion. you HAVE to cc them and then use c.r. formulas to come up with how much of a piston you will need. Since most of those came with greatly dished pistons, you could get to around 9 (8.8 was stated above) with a flat top with valve reliefs, then with that 268 cam it might still fly somewhere in the 400hp range or more. i took it straight out of jim hands book, the arp bolts on stock rods, etc. i did not port my heads tho. i dont have the confidence since i never did it before. the machine shop did put the largest valves on earth in it tho, and i dont really know if he pocket ported them or not. i did not ask as it is a long shot to whether i will ever see this motor run..
i think my heads were around 101cc's and with a 6cc piston i was going to get almost 9:1 on my 455. so with a 113 cc head, i think you will have to getta pop up piston. but i am just blowing this out my arse as it's been years now since i did the math. but that is what you have to do. as Cobalt said. there is a c.r. calculator at www.sdperformance.com
tho. and they are the people to ask questions about anything pontiac.
FOUND THIS THERE. ALL PONTIAC DUDE READ: http://www.sdperformance.com/moreTech.php?newsID=41