sorry i already modded mine by cutting flush with the outer surface of the head flange and sticking the tubes back in shortening the width your looking for. i did it to increase the clearance down on the subframe,and never had a clearance problem around the a/c box before modding. i'm using brp mounts on my 73 formula as far forward as possible 1/2-3/4 " clearance on the oil pan, about 3-4" to the firewall, pretty close to the a/c box with the valve cover and coils, plenty clear on alt to steering and steering to headers, notched the frame for the a/c. must have had that damn cracked block and tranny in and out 6 or 7 times.
you're going to have to modify the collector on the drivers side to clear the frame, just cut around the collector (but not through the tubes) on 3 sides and straghten it out, then get it rewelded. it really helps having a empty block and tranny for mockup purposes.