heads, how much hp gain??

[Unit 5302]

I have found Ebay motors to be a ludicrous place to buy the GT40 Irons. The old used sets routinely finish up over $500!!!

Find a set of used GT40's from a swap meet, have the cleaned up and put a little porting on them. The GT-40P's will require new headers as you have noted, I just didn't know if you had any mods. If you didn't, you'd be buying a set of headers and it wouldn't matter anyway

Most 180cc+ intake port heads are going to kill lowend.

As far as the alumium vs iron debate. Iron will be more reliable when it comes to gaskets, and fit. They will make more power than it's twin in aluminum. They do not allow you to run as much compression or timing as alumium heads, and they are much heavier than aluminum heads.

It's your call, but since you seem to be wanting to stick near the $550 area, the irons should be what you are looking at.

GT-40 Ported
GT-40P Stage 1 (requires new headers)
Windsor Jr.

Those would be the one's I'd recommend.