For those of you that have owned 914's in the past and are familiar with the website 914world there was a gentleman with a last name of Engman ( now deceased
) that manufactured what was called " the Engman kit " to help strengthen the tub on 914's . Many called it a clam shell but basically it was a very heavy duty L shaped piece of metal . It was aprox. 1.5 " wide at the top and sat on the horizontal portion of the interior rocker structure . It was aprox. 3 - 4 " deep and that portion went against the vertical portion of the rocker . The L pieces had many holes drilled into them and that's where you welded them to the rockers . The pieces were long enough to go from the door posts at the front to the firewall . It also included a piece that went on the bottom of the firewall .
When all 3 pieces were welded in they provided a big increase in chassis stiffness , I installed one on my GT clone with a 3.0 911 engine and the difference was VERY easy to feel . Now to be fair we also are talking a chassis that was designed and built 40 + years ago . But the principle may apply to the Boxster chassis also .