A six-speed conversion is not a standard operation so there is no book rate on it. I would not expect any shop to do this except on a T&M basis (time and materials - whatever it takes them, they charge you). Not a lot of people are doing this, because it's usually so much more expensive than gearing swaps (short gears and/or R&P) that can give you almost as much benefit.
A clutch replacement typically takes about eight hours of labor.
Labor rates vary around the country; you will have to ask your dealer or shop what they charge. Chicago-area dealer rates are around $90/hour, sometimes discounted. Acura of Brookfield (WI) charges $80/hour.