911 2.4 S Cyl Heads
 

I need to replace my 2.4 S cyl heads PN 911 104 307 1R Aug-1972
I purchase a old set of cyl heads PN 911 104 302 0R Sept-1971 the camshaft cover is the same number on both heads.
I assume that the 1971 heads are for a 911 2.2 S .
Do I have to modified the 1971 heads to fit on my 1972 car ?
Are the valves the same size on both cars ?
Do I need to bore only the heads contact surface to the cylinder liner , since the 1972 car is fitted with pistons maybe with bigger bore ?
TKS Marco