You should take off your stock bumper in order to install one of these rollpan bumpers and give your vehicle a smooth line finished look.
Most people have the ability to install these on their own with very few tools. Cutting, sanding and painting are usually what is required to complete the installation of a custom bumper.
Most likely they won't be illegal in your state, especially if you have an older model truck. You will want to check with your DMV to be certain.
Your local law enforcement will usually leave you alone with this customization, but you should learn you local laws before installing a roll pan.
With your new custom rollpan for chevy installed and your old bumper removed you should look at what comes next.
You can get great looking custom taillights by using a tailgate skin or sometimes they are cut right into the rollpan. You also can get custom license plate holders for your vehicle.