Cpp frame stiffeners don’t fit. Suggestions?

[Slowguy]

I purchased some cpp frame stiffeners for my step cut inside the frame. I’ll weld the steps as well as the stiffeners to the inside of the frame. But the cpp metal doesn’t fit inside the frame channels. Looks to be about an eight inch too large. Any tricks to get it to fit? Maybe I’m just overlooking something? I can’t see how something as simple as a u shaped piece of steel would be difficult to fabricate to fit inside a frame so I feel like I’m just overlooking how it gets installed.