If you wish to get around then "external balance" parts interchangeability issues as mentioned; convertor swaps later, just add a 120 gram weight to the flexplate prior to balancing, then use the "0" , neutral balance, steel crank type convertors.
Cast cranks are fine at the 500 hp level, however it's not really the material, so much as the RPM the engine will see that kills things.
To explain; it's the torsional stress "end to end" that eventually kills the cast clunk-sticks. By being "externally" balanced, what you're really saying is we're adding a weight to each end of the crank, outside of the motor, on the balancer and Torque convertor, to make up for "too light" of counterweights internally on the crank, given the cast materials less dense weight as opposed to the forged.
These same weights, at certain points in the rpm band, tend to set up a damaging frequency "end to end", as the crank "windes" and "un-windes" while rotating in the motor.
This is why It's always preferable to have the weight "as close to opposite" of what it's countering, as in the Forged cranks.
Long story short. 1 st damaging harmonics(the minor), on a ~2400 gram 440 bobweight occur @ ~32-3400 rpm
The 2 nd, and worst phasing(the major), will occur ~ 67-6800 rpm.
Build to 500 with the cast, get it balanced, stay below 6800 rpm, no worries.
JUST MY OPINION, no flames wanted, use whatever you like, forged or cast.
