[olddog]

I did some internet searches on the C6AEU heads. Any idiot can type anything on the net, so take this with a grain of salt.

Most sources say it is a 390 GT head.
Some said emissions head.
Some said it was used on everything from 352 to 428 standard - not a cobra jet.

The most credible sounding source I read said it was a 390 GT head used in the smaller chassis cars, like Mustangs. The 16 or so holes in the exhaust ports was used to allow different manifold designs, so they could be installed with the extreamely tight shock towers.

Not one source said they are 428 CJ heads. From what I read long ago, I would agree that they are not. They are likely OK heads for someone wanting a stock looking engine.

As for could you get to 500 Hp with those heads, with serious port work, yes. With mild port work and a radical cam, yes, but I don't think you would like its manors.

Frankly I would be more concerned about cracks and valve seats that cannot handle unleaded gasoline. Then I would be concerned about some of the ills of 1960's heads. Hot spots and engine run on. Pinging from poor combustion chamber design. I would definitely have an expert builder look them over and advise me, before using them.