I have a 390 i am currently rebuilding and wanted to put my GT heads on. Without cutting off the old header flanges how could i go about modifying my flanges to make them work, gts are the 14 bolt exhaust current header flanges have the two vertically opposed, just wondered if anyone modified before????

 

GT heads made for the mustang, cougar and fairlane which has the extra diagonal holes, have the same vertical holes. You do not need to modify anything.

 

Not always true. Normally the GT heads with the 14 bolt pattern (C7-C8) are missing the lower bolt holes on the 2 center exhaust ports. The heads can be drilled and tapped for the bolt holes but have to be carefully done. Or you'll see water. That said depending on which casting number they are they are somewhat sought after in the 14 bolt pattern to some restoring GT S, X code and 67 Shelby cars. Later service heads all had the 16 bolt patterns to cover all exhaust patterns.

 

Ford casting ID numbers and part identification info for Mustang Shelby parts shows pics of the C7 and C8 14 bolt heads under the heads section to see the differences. That said the GT/Shelby C7-C8 heads are not all that special nor different (other than the exhaust pattern) from other C7-C8 heads. In fact if you currently have the C6AE-R heads they are a better option IMO. Unless they are beat to death with issues. But please let us know what casting numbers of the heads are to better answer your Q's. C8's also have a lowered exhaust port which can be a problem with sealing some headers.

 

He needs to look at his heads and see if he has all the vertical holes. Or he needs to purchase some 428cj/sjc heads like the below.


2008_1104Image0003.jpg picture by Maxtor427 - Photobucket

 

i have the c6ae-R heads and my current heads and headers have the two vertically opposed bolt holes and the GT heads have the 14 bolt holes outside ports have 4 bolt holes and the inside two ports have three bolt holes. I was told someone may make an adapter?? but i dont know i have not found anything on the Web.

 

On my 390 i have the same number heads C6AE-R off a 66 mercury cyclone GT i had to modify the heads to get my headers to fit i drilled and tapped the holes and stuck a bolt in the ones not needed and cut it off and smoothed it down flat so it would all seal i have had no problems. but do remember dont drill to deep you dont wanna hit water!