Two words: THIN AIR. Actually, you don't really give us enough to go on. Is this all that you have done to the car? Have you dumped the codes to see if there's an underlying problem? Just the change to the GT-40 with a 70mm T/B shouldn't have killed things, although the 70mm is too large for a mildly modified or near-stock motor. You could always try the reverse diagnosis and start working backwards. Replace the Pro-M with the stock unit and see if things improve. If not, add back the stock T/B and see if things improve. If not, swap intakes...etc. I'd think that something else is wrong though. If you're in a high altitude region, that may be as far as you need to look.
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Jeff Chambers
Trophy Stock #3
11.97 Seconds / 114.5 MPH
Chambers Racing Team