Don,
When I was first doing my Brookville body, I noticed that the rear part trunk area seemed to be loose. The first thing I did was to pull all of their bolting and either replace with gr5's or at a minimum, a good gr5 lock washer. That stiffened it up some. Then I tied everything together with some 1" round and square stock. Needless to say, there isn't any flop. I also tied the cowl sides together with some 1/2 square tubing and most of the cowl floppiness is now gone. Then with the doubled flooring with 1/2 square tubing reinforcement and tabs for the seat, you can even lift the body with the doors open and it wont bend.