The carbon fiber tank you are thinking about does mount where the stock coolant overflow tank goes. It has 2 chambers. One is for coolant overflow and one is for some sort of fuel venting or overflow I have not got it figured out yet.

The coolant tank that goes in front of the steering head that is non plastic is the old corse part. From the early 90's 916/996 vintage bikes, they went to plastic in the late 90's. It is a stainless steel part with a sight glass from an 888 right side case. It takes the stock filler cap. Very nice part if you can find one. I think it was made from stainless because at the time they were trying to add weight to the race bikes.