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I'm reading bits and pieces of feedback regarding sparkplug wires, headers and clutch linkage problems with guys using these heads. Are these heads ones that I should stay away from due to how they fit in the first generation Mustangs?
 

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I'm running a GT40P in my 66. I did quite a extensive write up. Here's what you need to be aware of. The plug angle is more perpendicular and will cause issues with a lot of headers. On the driver's side if you can get or use a deep socket on the rear bolt of each header tube, you're fine. It means you have room for the spark plug. I'm using off the shelf MAC longtubes. While not designed to they will work. You need to use wires with 90° ends. I recommend the MSD Streetfire wires. They have a nice compact boot. These headers fit great with the clutch linkage, starter and power steering ram, no drop bracket required. As far as a Z bar, almost all roller block motors do not have the boss for the Z bar stud but a bracket specifically for this is sold. Stock exhaust manifolds with some minor grinding will fit. The one catch is that they won't work with the Z bar bracket. The inner stud for the exhaust pipe hits the bracket.

I believe Doug Thorly try y headers will fit. Someone on the forum recently bought a set. While examining them, they look like they will work.

The P head is the best flowing Windsor factory cast iron head. They also have a very efficient combustion chamber and need very little timing. Limit total mechanical timing to 28°-30°. They have a 60cc chamber which is very close to the 55cc of the typical 289 head. It's about a half point loss in compression but not enough to worry about. The improved flow of the P head will more then make that up with it's improved air flow and filling the cylinder up.
 

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+1 for everything @Huskinhano wrote.
Got mine dirty cheap from the junkyard, pulled them and paid $45. A few things you should keep in mind. The springs are not the best, if you want to build a high rev engine you should change them. There are spring kits from e.g. alexsparts.com available. Not much porting needed IMHO, you can work a bit on the exhaust side to remove imperfections. If you are afraid of the compression let them mill down a bit. Keep in mind, if you need a machine shop cost can go up fast. I don't know the goals of your build, but I would also recommend a different cam if you still run a stock cam. Intake manifold and carb should also fit to your goal.
I switched from stock exhaust manifold to the HiPo exhaust manifold. No problems with the spark plugs, but also the HiPo Z bar is needed.

Overall great heads, especially if you get a good deal. If a lot of rework is needed (resurfacing, valve job, new valves) get a quote from your machine shop first.
 
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