I agree with the external balancing, wish they were internal balanced. As for the cast crank, no sweating that. People going 450-500 hp for years and never hurt the crank. Some have taken the cast cranks to 9's in the 1/4 without failure.
No real problem with a cast crank from Chrysler. I only have one cast crank, from a 318 magnum, all the rest are forged. It may never get used, but it might be easier just to use it with the Magnum as a throw together engine.