At what point does a forged crank need to be considered essential. I am going to build a 393 or 408 stroker. I like the 393 but I only see forged pieces for the 408. Can the Scat 9000 crank hold up for an occasional 6300 rpm spin?
289 and 289 Hipo cranks were of the cast variety and can turn 7k without failure. I don't really think RPM is the issue, it is more the load when your engine is making maximum torque and trying to accellerate your car.
I have never had an engine potent enough to have a crank fail, so others here will have more insite.
I ran my 351W (Lightning block, rods, and crank) up to 6,800 RPM a few times, until I thought better of it. According to the Ford guys the two bolt block isn't strong enough without bottom end bracing; apparantly the crank is OK, but I wonder about the large diameter main bearing at sustained high rpm's.
Good nodular iron cast cranks are plenty strong. Don't worry about it unless you plan a blower or serious nitrous (over 125 hp shot) use.
Don't worry about the two bolt mains - they'll do fine up to 600 hp. The large diameter mains are only an issue because of parastic drag (eats HP) and oil heating. Do the math - the surface speed isn't that much higher. You weren't planning to drive over 7K for longer than acceleration time anyway, were you ? In high gear you'd be doing, oh, 140 or so... Kind of gets the attention of your local P.D. big time..
I know of a local racer that used a SCAT 9000 series crank. He made over 1600 passes in the mid 10 second range before the crank went south on him. I'm not sure what caused the failure. It may have been one too many shots of the juice.
I've pretty much decided to use the 9000 in my 331 buildup this winter. I think it would hold up fine for your application too. Attention to detail during engine assembly cannot be overstressed. Do that and the motor should last you a long time.