Not an expert, but ..
GT-40 - Old school iron heads.
GT-40p - Newer school iron heads. Better plug placement and needs correct headers because of plug placement. Smaller CC than the GT-40 for higher compression (if you need or want more compression)
GT-40X - Aluminum heads
Check here for flow numbers
Flow Page - Good Stuff
Good luck.
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Bobasaurus
1987 LX 5.0 - Engine Rebuild now installed - 26290 Wonderful Miles so far
Items now installed from the garage : GT-40p heads, Explorer Intake, FMS "P" headers, Crower 15511, SFI Balancer, 24#, SPEC Stage 1, T5z, 65MM TB, 77MM Pro-M, 255lph Pump
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