Ditto with a second body, but I'd have the 17-55 IS on it, and 70-200 IS on the other. In tight venues, the coverage gain from going down to 17 (instead of 24) is significant.
Get a used 5D (or a 5D2 if you want to spend $$$). Put the 24-70/2.8 on the 5D and 70-200/2.8 IS on the 50D. Should cover all your concert needs. Your 35L will also look fine on FF.
If not, get a 580EX flash.