Not sure what you are asking. You want to run 20" wheels with a 40 series tire that is about 12" wide? You 16 year old and have save $2200 to spend on your truck.
First off I am going to be an old man here and say save your money. Unless you have a rich daddy that is willing to drop phat cash for you to pimp your ride (doubt it since you are driving a 08 Frontier) learn not to be "ghetto rich".
Next 12" wide 20s. 305/40/r20 would fit the bill. Tire rack has 1. Runs $500+ for ONE tire. There goes your entire budget just for tires. When I was 16 I burned through a set of tires almost every year.
Stay in school. Study hard. Do drugs and drink beer with your friends. Have sex with girls (or boys if you are a girl or into that kind of things, I don't judge). But don;t waste away your youth by spending all your time working some **** job just so you can buy some shiny rims to impress NO one.
Sorry for the editorial. But if you want to mess with/learn more about cars. Find a used car for about $1K that is not running. Fix it and sell it for double.