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194 Posts
They're good, at least quite a bit better than a stock E7 and the GT40 head. The angle that the plug comes out of the head is different than most heads, it sticks almost straight out which will make most headers interfere with the plug, the MAC and BBK shorties & some other headers go pretty far away from the head before turning down and that helps, most people use 90 deg. plug boots too.

Heres a good link
http://www.stangpro.com/
If it goes straight to the home page go to the 'articles' section and scroll down to GT40P.

Intake Flow @ 28" H20

.100" .200" .300" .400" .500"
GT-40P 61 128 169 195 196
GT-40 54 107 157 183 192
E7TE 59 114 144 156 156
'69 351W 52 103 149 174 183
289 50 97 127 156 165
GT-40 (Y303) 64 114 167 199 213 (aluminum)
GT-40 (X303) 53 114 170 207 226 (aluminum)

Exhaust Flow @ 28" H20

.100" .200" .300" .400" .500"
GT-40P 52 90 123 135 139
GT-40 47 94 119 128 128
E7TE 42 78 105 115 116
'69 351W 48 77 100 113 118
289 37 71 93 104 107
GT-40 (Y303) 52 98 128 137 134 (aluminum)
GT-40 (X303) 50 106 139 153 161 (aluminum)
 

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9,250 Posts
My personal opinion is that they are not so much better than a GT-40, especially one with a mild port job, to be worth the header/plug hassle.
 
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