[Unit 5302]

Wow... $30/ea set or $30/ea head? Either way, those things used to be $500/set, minimum.

Port and polish
GT-40p Advantage = compression ratio. 59cc combustion chamber vs 65.5cc on the GT-40 Iron.*

GT-40p Advantage = low lift camshaft. If we were talking about an XE series camshaft with high lift, it would favor the GT-40 Irons as the Irons do flow a little more at .600 lift.

GT-40p Disadvantage = Exhaust valve size. 1.46 vs 1.54. There is only so much air you can squeeze through the smaller valve on the GT-40p heads.

GT-40p Disadvantage #2 = Headers. The p heads require special headers so I have to wonder whether or not your exhaust options will be as good once the porting/polishing is all done?

*Your compression ratio may get pretty high on the p's. On a stock 69 Mustang with the 351w, you'd be at about 11.0:1 on the GT-40p's and close to 10.1:1 on the GT-40 Irons. I don't know what pistons you're using so that makes it impossible to truly say.