I am curious know how Tom DiCenzo's got his 1963 MGB tubed so much I didn't see any details of the rear frame.
http://www.britishv8.org/MG/TomDiCenzo.htm
Dave
From reading the article it says that he used a home built 2" X 3" steel tube frame from the original front crossmember back. Probably welded the existing floor pans and crossemembers to that down the middle of the car and then moved the inner fenderwell in to match the frame rail in the rear. From the photos he obviously has a custom fuel cell and box mounted in the rear.
Seeing how it's a drag car, it's probably a standard "back half" job common with Pro Streeters & dragracers. Much like Bill described.
Google mini tub or back half tub.