Listen to Bob. I have seen dozens of these over the years & The new frame made from 2"x4" retangular tubing is the way to go. It's very inexpensive to build.
I have seen boxed T frames twist out of shape cracking the fiberglas body in the process. The original frame was made to twist & this lead to work hardening the frame. If you look at the frame carefully you will probably see cracks in it.