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Check out eBay and/or call your local salvage yards and ask what they want for a pair.
the D5 cast code basically says that it's a 1975 part. there's no way that head came stock on anything before late 1974, but the casting could continue well on after 1975. there is also a casting date cast into the head, gman26 says he found a "71" to which Brian S replied that it's prolly a "7I" meaning Sept of 77, but i don't think "I" is used in the date codes, it would be skipped to avoid confusion between the letter "I" and the number "1". a date code for Sept 1977 would look like 7J15, 7 is the year of the decade, J is the 10th letter for the 9th month because I is not used, and 15 would be the day of the month.
the 71 that gman26 found is prolly just a random number, there are a lot of them on Ford castings. but as Dennis Miller says,"i could be wrong"
You're right, it's not. I forgot about that one.
The D5 means it was originally designed for the 1975 vehicles. The actual casting date can be anywhere from late '74 up until the heads were discontinued. Heads are not redesigned every year so casting numbers usually overlap a few years.