Mike is 100% right, also remember that a minute difference in change in diameter will cause a big difference in fuel. .001 is significant and rarely will you change more than .002.

I have had to drill IFRs but it should be considered a last resort after all the things Mike said

When making changes, you should calculate the area of the passage and make changes based on the change in area (same as calculating the area of a circle A = Pi R Squared (Sorry couldnt get the formula to look right)

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70 Sportsroof, 427 FE/489 cid, 10.7:1,Ported Edelbrock heads, and RPM intake, 1000 Holley HP, Ceramic Hooker Supercomps, TKO-600, 3.70, A/C