Truck frame widths are measured at the the rear axle, outside to outside of the frame. It should be 38" or 42" if it is a normal truck that came with a bed from factory.
If it is a cab & chassis truck, which is a straight flat frame that did not come with a bed from factory, it will most likely be 34" outside to outside frame.
For aftermarket flatbed truck beds, the standard is 34", 38", 42" as well.
A lot of dealers like us will just keep the 34" and 38" frame width flatbed truck beds and install kits in stock, as you can use a 38" frame flatbed truck bed on a 42" frame truck.
If you have a 42" truck frame, and you choose to buy a 38" frame width flatbed truck bed from us, you may need to run some thick flat strap or square tube across your truck frame, then sit the flatbed truck bed down on it. Then you can use pieces out of the kit still to save fab work.
View the PDF files below to see the material that each install kit will come with.
Cab & Chassis Install Kit
Dodge 38 Inch Frame Install Kit
Dodge 42 Inch Frame Install Kit
Ford 2016 & Older Install Kit
Ford 2017 & Newer Install Kit
GM 38 Inch Frame Install Kit
GM 42 Inch Frame Install Kit
GM 2019-2020 Install Kit