i think your 7" has got an irregular length gas system. sort of like those 16" uppers they had a while back that were drilled between a carbine and a middy length on the gas system.
so you probably will get a bit longer gas tube than a regular 7" upper. not much of a problem really.
we will help you get it running if there is a problem. somebody has got a gas chart for the 7" barrel somewhere!!--LOL