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I have a 67 390 mustang and want to beef its power output. I have heard the exhaust manifolds are a weak link. Are 428CJ heads a better upgrade, or can I get what I need from the GT heads with some modification to them? Likewise, should 428CJ manifolds replace the GT units? Or could I Extrude Hone the GT units? I want the engine to look factory(no headers). I have a 428PI intake to add. My goal is to add 50 to 75 horsepower and significant torque above the stock engine.
 
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The PI intake is all you'll need, it's a great choice. The heads aren't very good but can be made acceptable. I've seen 352 heads flow as well as a good set of D0VE's for a 460, but it's work. Just as easy to get a set of Edelbrocks & paint 'em. You're exhaust manifolds are a hinderance too. I wouldn't touch them as they are getting as rare as hen's teeth! You'll have to be careful in the head choice or you'll have to drill the manifold holes. It's simple, just don't go to far.
 

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If you have the CJ parts available, a co-ordinated change would add some HP, but the amount would depend on the engine's current state of build.

The CJ heads have a slightly larger combustion chamber so compression would drop a bit and the valves are bigger, which would suggest the need for re-checking valve to piston clearance...more so if an aftermarket cam is being used.

My old Muscle Parts catalog doesn't show any differences in the 390's of your vintage, with the exception of minor HP differences between the regular and GT engine, which can be traced to the camshaft, with the GT having a split duration with a higher lift than the regular 390. This cam is also used in the 428 CJ, at least as a service part...

You also might try posting a query here if you haven't already done so.

IME, asking for a 15-20% increase in HP from these changes is likely a bit optimistic...possibly, with an addtion of the CJ camshaft and some tuning refinements, maybe...

Good luck!
 

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As they say..."Speed costs money, how fast do you want to go?"
The Edelbrock heads are a great upgrade as are headers. I've got Hooker Super Comps and they hang too low for worry free driving over speed bumps. I've seen FPA try-Y headers for FE motors and they tuck up nicely inside the frame rails so ground clearance isn't a problem. Next time around I will try them. An aluminum intake manifold saves weight and may also aid your quest for more power. For more insight check out the following site

http://www.network54.com/Forum/74182
 
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