GT40 Heads:
Looking at FRONT of each head (if standing in FRONT of motor), on the end of each head, you will see (3) vertical "bars". You do not have to remove any valve covers, or any other parts to see these markings.
Looking at the TOP of the heads, at the front corner (top, outside and near bottom corner of valve cover - towards water pump) you will see the letters "GT" stamped into the head casting. You can also see the same cast letters near the head's first bolt hole (again, closest to water pump). You do not need to remove the valve covers or any other parts to see the casting letters.
NOTE: The standard GT40 heads were used on all 1993-95 5.0 Mustang SVT Cobras and up to the mid-year 1997.5 Ford 5.0 Explorers.
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GT40P Heads:
Looking at FRONT of each head (if standing in FRONT of motor), on the end of each head, you will see (4) vertical "bars". You do not have to remove any valve covers, or any other parts to see these markings.
Looking at the TOP of each head, at the front corner (topside and near bottom corner of valve cover - towards water pump) you will see the letters "GTP" OR “P” stamped into the head casting. You can also see the same cast letters near the head's first bolt hole (again, closest to water pump). You do not need to remove the valve covers or any other parts to see the casting letters
GT40P's will not have the emissions crossover ports on the back of the head for the steel emissions tubing for smog pump/exhaust.
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As to which is preferred, GT40’s are the most common and usually the preferred head due to not needing “GT40P” specific headers. Trying to find GT40P specific headers TODAY is a PIA because most Companies that did make them have since closed shop OR have stopped production being there’s no mass demand for them anymore. You could find them used, but it’s like trying to find a needle in a haystack.
The problem w/ GT40P heads & header issues, lies mainly on the passenger side head due to the severe spark plug angles, with the worse being plugs # 3 & 4. You will need to get yourself a set of ignition wires that have a 90* boot, instead of the common 45* boot. You will also need to get High Temp ignition wire boots (non-flammable & heat resistant up to 1200*), because if you do not, you will be burning up ignition wires.
Even with using the specific GT40P headers (which used to be made by Ford Racing or MAC), changing out the plugs is still a PIA because of the spark plug angles.
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There’s this excellent archived web page that has the complete details on GT40/GT40P heads:
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Good luck with the sale - definitely an excellent asking price for all that is bundled.