If you look closely you will see that he mounts the self made rear upper deck NOT to the shock tower base, but to the rear bulkhead (BX-302RL and BX-302RR). Doing so the upper deck moves higher (with the help of some bushings in the front and rear) and the belt is entirely under the deck. You can see these bushings in the front (silver) quite easily at the end of the video.
I mounted a stiffer rear shock tower on the BX after a crash landing. After that the shock tower didn't break any longer, but the rear upper deck started to break (not once but n-times
). Actually the BX is the first car I ever had which breaks the upper deck quite easily. The good thing is that Billy Easton seems to have experienced the same problem
. The reason seems to be that the impulse of a crash landing is directly moved forward from the shock tower to the tower base and finally to the upper deck, which might break if the shock tower did not (as Bastos already mentioned). With this mod the impulse which goes into the upper deck is reduced since the flex of the bulkheads should be less than the flex of the tower base...
Hope someone understands what I mean (english is not my mother tongue)?!
Cheers,
Jens