The crank bolt for the crank mounted fan pully on my '41 was installed without the Bellville washer which allowed the fan to pull the pully forward to the point that the fan destroyed the aluminum radiator. Made quite a racket and doused the engine with coolant. That killed the engine. Not sure if that was from water getting into the distributor or all over the coil and spark plug wires. After an hour the engine dried off enough to start, but still needed a rollback to get home.
Another time I had a serious boil over blowing water and steam all over the engine, which died. After an hour and some water in the radiator, it started and ran normally.
I have since replaced the ignition wires with copper core and new boots, but have never run it in a very hard rain.
With the crab being widely preferred and thousands of them out there in all kinds of conditions, I can not imagine that a crab with properly installed cables and boots will have problems.