That looks like the rocker panel to me. Those are available from the usual vendors, as is about anything else in that area. I replaced both of my rockers. Alternatively you can buy a complete floor which has the rockers already welded to it along with the reinforcement shown in your third photo. Take a look at the photo link in my signature for how I fixed my cab. Start here and click on the left arrow.

Last edited by klhansen; 07/08/2025 3:59 AM. Reason: ADDED PHOTO LINK

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.