With this many questions about tube lengths/fitting/etc. , you might be well served by doing one of two things:
#1. Buy an upper that is already assembled. Barrel, receiver, gas tube, FSB...that way there are no parts mismatches. If you're assembling parts that you bought from various people and the whole she-bang doesn't...well...go "bang", then who do you ask to "make it right"?
#2. Find a smith/gun shop local to you that knows about AR's and get them to guide you and perhaps assemble your parts.