Just beat the Ford GT yesterday with a GTR'07. The important tune is down-force, which I missed in the past since the aero kits were not sold in tune shop but in GT auto. Before getting the aero kits, I, like you, was 7-8 sec behind the lead car; Once installed, I win with a second try, lap time in 1.27+
The rest tunes you can easily find in the forum. However, those tunes made others successful don't necessary fit with your driving style.