A mild steel is plenty. The frame is probably made from it anyway. This will allow you to bend it and cut it easily.
Short Answer?
Same Stuff... and Just as Good.
I agree, I've always used mild steel for frame repairs however, GM is always touting their "high strength steel".
>>The 2015 Silverado’s strength can be measured through five core elements:<<
1. Fully Boxed Frame
Silverado’s fully boxed frame – including main rails and major cross members formed from
high-strength steel – provides a rigid base for maximum hauling capability, excellent crashworthiness and the longevity truck customers demand.
The frame comprises three sections: front bay, mid bay and rear bay. The front frame bay uses hydroformed side rails for optimal strength and lower weight. Pioneered by General Motors for full-size trucks 15 years ago, the process uses highly pressurized fluid to shape the frame rails, making them stronger and lighter than comparable welded multi-piece components.
The side rails of the frame’s middle bay are constructed of two pieces of
high-strength steel welded to form the sturdy box design. The front bay’s hydroformed rails slip inside the boxed sections of the middle bay 9.4 inches (240 mm), creating a bend-resistant rigid structure, while the rear-bay boxed rail sections are joined to the middle bay with an overlapping, shingle-type construction. A frame-integrated hitch enables maximum trailering capability right from the factory.
Engineers validated the frame’s strength in the lab and in the field. In the lab, Silverado prototypes were mounted by their wheel hubs on a simulator that shook and twisted them with extreme intensity. Outdoors, they were driven over a variety of rough surfaces and swells – including torturous “truck blocks” – inducing natural frequencies and real-world driving conditions that don’t occur in the lab.