A few years ago I saw a TR6 race car without a cage or any obvious additional strengthening that handled like it was on a rail. The only observation was a series of large pop-rivits spaced pretty closely thru the floor to the frame (the rivits followed the shape of the frame) TR6 frame and 4A frames are essentially the same. I asked the owner if that was his method for strengthening chassis flex and he just smiled. A few years later I again saw the car at Watkins Glen and asked him again, this time he was a bit more forthcoming and nodded his head in the affirmative. Sort of created a unibody out of a chassis on frame construction. He again was very competitive until his engine went up in smoke.