"...6x-4 heads which has a 87cc[?] chamber. The 6x-8 was a 455 head which is ~100cc.."


Just to keep all the facts straight, the 6x-4 heads are said to have chambers closer to 93-94cc.

The 6x-8 heads were used only on 400 engines, NOT 455's. They're supposed to have aprox 100-101cc chambers.

http://wallaceracing.com/head1.htm

http://www.classicpontiac.org/FAQ.asp?myPage=V8HeadInfo

To know your actual chamber size FOR SURE, they MUST be measured. Size from the factory can vary, and the chamber volume could have been changed by previous head work.