I am not sure,but is there a rib hanging down in the old camcovers that would hold the rocker shaft in, were it to break ?
Nope, none. Racers who used tall cams or those who over-revved often enough to break the shafts used several methods to hold the half-pieces in the towers: one was to stake the ends in with a punch (which also inhibited their rotation, losing some high-RPM power in the result), another was to drill and tap (and safety-wire) some very small bolts and washers over their ends, like is found in aircraft engines (and probably where it came from?). I also saw some that were drilled and pinned with roll pins: Honda sort of used this approach, but easier to disassemble. The loss of rotation DOES make a noticeable before-and-after top-end feeling in the engines, though. They rev more freely to 8500 RPM with the moving shafts, and the shafts never wear out. The locked-down ones must be flipped 180 degrees at about 40k miles to stop the 'clicking' sound they otherwise generate.
I also saw (just one) drag engine where the builder had made little aluminum blocks in the cam cover that reached down to stop a migrating half-shaft from moving out of the tower (in the center 4 spots). I thought this was an elegant solution, even if it was done clumsily.
The outer end of a broken shaft only moves a short distance before it hits the cam cover and stops: this isn't trouble, as the rocker still works and can even go unnoticed. The inner half-shaft, if it starts sliding, can reach the cam sprocket, which can cause more significant problems.
