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Ok....I need a speck of clarification on the issue.
I know the GT40-Ps will bolt directly on my 289. And I know to keep the cc's down to about 55,
they have to be machined slightly. But, do I have to change the rockers or is the change
a preference? Aaaaand.....what length pushrods would be needed...or does that have
to be determined after the machining?
 

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Yyes, you'll loose about a 1/2 point in CR if you don't have them resurfaced from 59-60 CC's to 55 CC's. I've spoke with another VMFer who also used the P head on his 289. He (Matty)told me that he wanted to have the CC's reduced but forgot to do so. Anyway, he said that they still work very well.

Pushrod length will have to be basically determined by how much the heads have been cut. But the best way to determine is to check geometry and go from there. Mostly because the block could have been resurfaced at one time and as built specs vs design specs from the factory can be 2 different things.

See my link below for header option.
 

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The GT40P's are set up for the screw-in towers rather than the screw-in or press-in studs. You will probably have to convert over to that style of rocker. At least I did when I put them on my 302. The Ford Racing roller rockers are what I used. I'm still screwing around with the proper valve train geometry for my engine. I'm running the same pushrods I had with the stock heads but the rockers are very noisy. May have to machine the bases of the towers to clear that up. I believe the chambers are 58 cc's which are the same as my 69 stock heads. The GT40P's pared over .5 seconds from my quarter mile time.
 

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I had mine machined .025 which is suppose to be the magic number. I bought Crane cams roller tipped rockers #11746-16 and because they're studs and not pressed in I had to get a conversion kit for the rocker arms also Crane part # 36655-16. After it was all assembled we measured for push rod lenght, which was real close to 7.0" I don't remember exactly. Here is a link for the headers I bought http://www.stans-headers.com. good luck.
 
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