Resize your Windows 7 partition with Gparted. After you resize it manually, install Backtrack to the space you want. Grub should reconfigure itself (since you already had Ubuntu), and all should be well. I've done it before, and it worked for me. Gparted may take a while if its working on a big partition (took me a day for a 300gb drive) but it will get the job done.
P.S. You may have to resize the Windows 7 partition multiple times in increments of 10gb, when I tried it wouldn't work with more than 10gb at a time.