I assume that it is a stock C5 with an automatic transmissions????
If this be the case, you either are starting with a 2.79 or 3.15???
Either case you can expect top end to be reduced. I know of only one group that has tested an automatic tranny with a 3.15 rear axle. Top end was 161mph.. Moving to a 3.42 will drop top end to around 150mph give or take a few miles per hour.
This is based on the other groups numbers. Hope this helps you.