My money is on the LS1 because of how few of mods you listed and because it is much easier to make a mistake with an m6. When you say exhaust I assume just a cat back since you did not mention full exhaust. If you are a very good driver, have full exhaust and some dr's I would give the win to you assuming the ls1 is stock.

 

+1 for the LT1 U should beat him pretty easy. I have a 93 M6 Camaro W/ Shorties & exhaust 268rwhp 302rwtq I raced a 98 Trans Am A4 W/ Long tubes, Exhaust, & lid. Pulled him pretty good

http://www.youtube.com/watch?v=vDktQMdzVWw

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I think you have a decent chance. It depends on the cars really. They all seem to run differently. Do you know what gears the LS1 car has (2.73s or 3.23s)? Have you or him ran at the track?