I did from the junkyard and one of them was cracked so now I’m debating again lol!
Dang, that sucks, but I guess its the risk of junkyard stuff.
To answer your question, and to be very direct, the differences on these two heads has been covered vastly and in depth since people started using them in the 90s. Here's an excerpt I found that is over ten years old researched and written by Kell Bean:
"When it comes down to making choices for your heads, it’s important to decide where you’re going, and what you need. The GT-40 Iron heads do sport some enhancements beyond the P’s. For a forced induction combination, the Iron’s do have a lower compression ratio, and in the case of more aggressive camshafts the Irons usually came with GT-40 valvetrain, good for about .520 lift while the stock P head springs only go to .500. If you were to use an XE series camshaft with a great deal of lift, the GT-40 Iron head would probably work better than the P head for pure, peak flow due to the more linear flow characteristics the Irons have continuing past .500. The added advantage is the GT-40 heads work with common headers, and the P heads require specific, and more expensive GT-40P headers in almost all cases. The GT-40P head; however, has it’s advantages too. A more efficient combustion chamber design delivers a better burn, a little more compression for more power in a N/A configuration, more peak flow at the valve lifts most combinations involving these heads will experience, and a superior area under the curve. By under the curve, I mean the flow rate viewed as a graph. If you take the horizontal X axis, and label it "Lift", and the vertical Y axis, and label it "Flow" plotting the flow curve out between the heads would be interesting to look at. It all adds up to a definite advantage for the P head in most applications."
GT-40P
Material: Cast Iron
Combustion Chamber: 59.0cc
Intake Valve: 1.84"
Exhaust Valve: 1.46"
Peak Flow Int.:
[email protected]
Peak Flow Exh.:
[email protected]
Stock Max Lift: .500"
GT-40 Iron
Material: Cast Iron
Combustion Chamber: 65.5cc
Intake Valve: 1.84"
Exhaust Valve: 1.54"
Peak Flow Int.:
[email protected]*
Peak Flow Exh.:
[email protected]*
Stock Max Lift: .520"*