1997 5.9L Magnum Roots Supercharging and Fitment

I pushed a 318 to 10 psi on e85 with stock internals (upgraded fasteners) with a turbo. They may be right considering the load on the crank is significant. Forged crank would fix that. Pistons and rods are highly questionable at 15-20 like you're thinking. But it's all in the tune