I'd say ~9:1 CR for regular 87 octane gas/open chamber/no quench. That'd let you run a quick curve. To answer your Q you'd have to plug in all the required specs into the KB calculator to see what you're at. EDIT I reread & are you sayin the 667's are in there now & you have 906/452's now or do you have 915's & are considering 906/452's to lower the CR? same deal you'd wanna pull the heads & measure the deck height & could lookup the nominal CC's of the 906/452's & plug it all in & go from there. The thickness of the outer head bolt hole boss is within a few thou of 1" on unmilled heads and curved swirl marks on the deck tell you that the heads have been milled (or not) so you may or may not want to CC em for an accurate CR number. That in turn will tell you how thick of Cometic to order & possibly a regular blue Felpro will be enough. I'm assuming your timing (initial/springs) is in the ballpark