Hey Rusty......
I too am interested in this, as much as I'd like a
TC Design cage, I don't think I
need to go that route (cost, and turning the 914 into a daily driver, I need and want to be able to easily get in and out of the car....)
Hopefully Tony C can chime in on this to see what his professional recommendations are to stiffen the car up without going all out with a cage.
I've heard of some people welding tubes(round and square) along with sheets/plates of steel to the inner tunnel and the outer longs to help stiffen it up.
It would seem that almost
any additional reinforcment would help, but the key is to optimize the designe and placement.
I thought Brad said the cars flex the most right at the floor area below the mirror on the window.
HTH
Mike