Yes and no, both good answers depending on the rpm at which the horsepower is transmitted. At 10,000rpm. I am sure it would withstand the torque. But at 5,000rpm the torque necessary to generate 500hp has doubled.

Now to put into practice. The original load for this flexplate was max torque of a 5.9 Magnum engine, net of 345 lb-ft. Add the extra cubic inches of a 408 and using the same ratio we get 391 lb-ft. The factor of safety for this part has to be at least two, I'd use three if I was designing. We've increased the torque by a factor of 13.3%, well within the limits.
My conclusion is the flexplate will work, no problem.

Others will ask, if the crankshaft assembly is neutral balanced, what difference does it make if you substitute flexplates?
Answer: If it was indeed internally balanced neutral, it shouldn't matter a bit.

R.