You are aware that by going to a 14.5" Barrel it becomes a Short Barrel Rifle according to ATF rule and requires registration and a $200 Federal Tax Stamp. Once an SBR always an SBR. The exception is for example if you would have the 14.5" Barrel with a Vortex Flash Hider permanently affixed to the barrel. (Pinned & Brazed).
My suggestion unless you have the proper tools like a torque wrench, barrel nut tool, receiver block, head space gauges and etc. I would just buy a complete upper with the bolt carrier group already in it. This will also assure everything is correct.
The Constitution is not an instrument for the Government to restrain the people. It is an instrument for the people to restrain the Government!
Today, we need a nation of Minutemen, citizens who are not only prepared to take arms, but citizens who regard the preservation of freedom as the basic purpose of their daily life and who are willing to consciously work and sacrifice for that freedom."
-- John F. Kennedy