Modified vehicals can be a problem with your insurance carrior. If your mods are items like adding a winch, bumper, topper or aftermarket radio, then most carriors will cover them at time of loss IF you have advised them of the mods and their cost BEFORE the loss. They may charge you an additional premium for the added value of your truck. Some changes like large HP increases or lifted suspention could cause you carrior to cancel your policy due to a perceived increased hazard. All policys will exclude coverage while a vehical is being used in an organized race.
You will most likely do fine if you talk to your agent about your specific mods and cost. Remember that most policies are on a actual cash value basis. This means that for a total loss you will only receive the depreciated value for your rig. There is such a thing as a stated value policy. This type of coverage allows the carrior and the insured to agree upon the value of the vehical before the loss and insure it for that amount. The availability and cost of a stated value policy will vary by state and carrior.
After discussing your needs with your agent, ask him/her to give you a brief written explaination of how you mods will be covered in the event of a loss. If they refuse to put it writting or simple say "don't worry your covered", then find yourself a different agent/broker.