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Do a search on flare side wood floor(s) but I remember seeing something you need 2 sheets as the bed is wider than 1 sheet.
The post gives measurements to cut the sheets IIRC.
Dave ----
Basically, you could use four factory 1x6s and four 1x8s, as the bed is a bit under 56" wide. With Plywood - In the thread above you'll see that you need two, 4x8 sheets cut into three pieces total, one in the center and the two outboard ones handling the bed mounting bolts.
On my truck the wood was 6 feet, 5 1/2 inches long (77 1/2")
I recommend you measure your truck specifically though just in case
An important note - you want the joint between the middle section of plywood, and the two outer sections, to fall right where the bed strips are. That way the joint is hidden.
You're locating those joints at the bed strips which are just inboard of the bed mounting bolts. That way you can easily mount up the bed with the outboard sections, and put in the center section afterwards.
OK, so I went and dug up my templates. The middle piece is 22 1/2 inches wide by 78 1/2 inches long. The two outer pieces are 15 3/16 at their widest point. I then suggest you set the side panels and draw your lines to cut the wheel wells out with the jig saw or tool of your choice. I hope this helps.