67 273 heads flow about the same as late 360 heads, up to .400 to .500 lift about 180 cfm vs about 190 cfm. But they have the small ports so the Performer will work. There is more potential with the 360 heads. But the 66-67 273 heads are not as bad as one would think considering the compression boost. From 73cc to 65cc unmilled, or 64.7cc to 57.3 milled .040" and .038" on the intake side.