At first I figured the C3 casting would be a 260 head, but if they have 289 stamped on them, then their 289 heads, and I would ASSUME they have 54cc chambers. I know that some of the earlier cars (before 65) had even smaller chambers than this, but I think they were HiPO heads.
If no one here can give you a definate answer, try posting over at vintage-mustang.com. They have lots of people who are very good sources of early Mustang info...especially the stock stuff. A couple of regulars are MCA judges and really know their cars. If they don't know it, they can sure find out. Are you planning on using these heads on 5.0?
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66 GT Fastback 5.0, 5-speed, cable clutch, JBA's, 3.40 gears, EFI coming soon?
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