I have a 1970 400cui in my 1971 Pontiac Firebird. Would it make a difference in the performance if I put a set of 6X heads on the engine? I have eldebrock RPM manifold and a 750 cfm (eldebrock) carb on it as well. Thanks for your help.  

Compared to your 70 400 heads, the 6x heads will have larger combustion chambers which will lower your compression ratio to around 7.6:1- 8:1. The original heads, depending on the application, may have smaller valves. How much performance you lose depends on what 70 heads you have. All d-ports flow similarly, so the loss of compression will likely make your throttle response a little soggy and drop your torque and hp slightly, you may gain a little or break even at the upper rpm range if the 6x heads have bigger valves. All 6x heads have 2.11" intakes, 1.66" exhausts, screw-in studs and guide plates. Some 2 bbl 400's used 1.92" intakes and 1.66 exhausts. In general you are going to lose power and throttle reponse with the 6x heads, assuming you don't change anything else, but it will run better on low octane pump gas, espacially if the original heads are high compression pieces (10.25:1 or higher).  

The 6X heads with the 8 stamp can be 100 to 105 CC those from the 350s were stamped with a 4 those are about 95 depending on who talk to.  

I have a set of 6x heads with a number 4 stamp on them. Can I use these on a 455 block with 030 overbore and gain the compression performance I need to run on high octane pump gas. It is my understanding that these heads were produced for the pontiac 350 and have smaller cumbustion chambers.  

I don't know much about cylinder heads, but I have a stock 400 out of a  '76 catalina that has 6x-4 heads on it. I thought I would let you know that they were not just put on the pontiac 350.  

Interesting, are you positive of the actual engine size, what are the date codes, block casting number and suffix codes from the block and the date codes from the heads? Could be that they've been swapped at some point.
Steve  

Yes the 6x-4 heads were installed on both 350&400 engines from 1976-79 and are 91cc's