Scott, so you're saying such a plate is not beneficial at all? Or, only in low horsepower applications? Not too many low horsepower applications in a GT40...
The plate will fix the problem with generated thrust forces cracking the center section, but with the unsupported 5th gear too much HP in the wrong situation can cause the pinion shaft to flex and strip out the 5th gear. I've seen it happen in 3 cases all with what I'd consider moderate hp levels.
If you look at the designs of the later Audi/Porsche/VW (01X,01E,02A) gearboxes, an extra bearing was added to the end of the pinion shaft to prevent deflection of the end of the pinion from occurring.