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From what I have understood, the cast cranks are fairly durable if ran at reasonable rpm ranges. Pistons are usually what fails when using nitrous.
The stock cast pistons are probobly going to let go before the crank does. I have only had it on one engine & it was just a 125hp kit on a stock 350, never gave me any problems. I'm sure some of the nitrous experts on the forum will chime in.