Need confirmation on stupid stuff

I can’t find the gun I want on the internet, so I’m going to make the cardinal sin of building it, buying the parts I want, knowing in advance I don’t know anything and will have a 92% chance of screwing it up.

That being said I need confirmation on rifle length gas systems and AR uppers.

I believe that I can buy any stripped down DI upper, and then buy a rifle length gas and only worry about getting a rifle length barrel to mate the gas ports up on the barrel and gas system.

to put it another way, there’s no such thing as a rifle length upper and I need to get my debit card ready for Memorial Day.