GM cast nodular iron cranks are stout enough for the majority of stock/super stock applications. On a 396, there is roughly 3lbs difference between cast and forged and for a 454, the wieght penalty is closer to 4lbs. I have seen numerous 468's with 2 bolt mains and cast cranks (bracket engines between 600 - 675HP) and never have I seen a broken crank.
BTW, 3 to 4 lbs off the crank is not insignificant in terms of performance. Maybe not measurable on a water brake dyno, but physics still governs our universe the last time I checked.
Take care guys.