What size chamber does E7'S and GT40 heads have? I think my 65 289 hipo heads are 54 cc right? how much compresion will I lose if I change over to e7's or gt40 heads and will the improved flow be enough to make up for the diff?
That depends upon which GT-40s you have. I just picked up and old '98 Ford Motorsports catalog and they listed the iron GT-40s at 65.5cc and the Aluminum at 64 cc. In general I think the E-7s and Gt-40s are all somewhere in the neighborhood of ~65cc.