There was an extensive discussion of this topic when I first got on this forum, which I thought was very interesting. So...
The good news is: you can get a GT40 body in carbon/Kevlar from Safir GT40 in Cinncinatti. They have done it from their molds and would do it for you. It is impressively light; two thirds if that the weight of my FRP GT40 body, I compared it myself. If you want to save weight, that is a lot easier and cheaper than a carbon tub. And a hell of a lot safer.
I agree with the above as far as the engine goes; if you are going to spend huge kilobucks to make a light rigid GT40 chassis, you should put in the lightest hottest motor you can. Every pound you save will have cost you a fortune; why throw the weight saving away in the engine room?
As for a carbon tub, I think it's a mistake. I thought, and still do think, that an composite tub for a GT40 is a great idea, but I think you could do the same thing, or very near it, with S-glass, some Kevlar, VE or epoxy resin and good prepreg or vacuum bag technology. OR, you could build the entire tub out of preformed FRP structurals which are easily available; the disadvantage there, though, would be that you couldn't as easily specify fire-retardant resins. Which you would want, but then it wouldn't be any more fire-resistant than a steel tub.
All of this is about getting weight out of the tub, in order to go faster, I guess. One thing that we haven't discussed is that there are now intermediary sandwich type materials that allow you to weld steel to aluminum. Possibly some weight could be saved in a metal tub by making some parts of aluminum. If properly painted, it wouldn't corrode, and would be just as strong if it were done carefully.