Basic upper build;

Install the barrel, the looking at the center of the top and bottom lugs on the barrel extension, compare to the center of the gas tube channel and center of front take down pin lug.  If the barrel extension is twisted in the upper receiver, then the bolts are not cleanly going to lock and unlock from the barrel extension. Also, using a drill bit, make sure that the top center between spines/ top hole in the barrel nut aligned with the upper receiver channel for the gas tube.  The tube needs to float between the barrel nut in the upper receiver channel for it when mounted, and not be bound up against the sides of the spines/channel


On the gas block, install the tube, and with the set screws removed, look up the threaded channel for the set screw at the back end of the block.  This lower channel is used for the back set screw and the gas passage from the barrel to gas key.  Here, you wan to make sure that the bottom openning in the gas tube is aligning with the block gas passage.  If needed, use a drill bit up through the channel to enlarge the side channel on the gas block.

Next, take a good look at the ID of the gas block at the gas passage channel.  If a dull bit was used to produce the channel, or tap run too deep, then it can peen the entrance of the gas passage channel to leave a high ridge around the entrance.  If such is found in the ID of the block remove just the ridge with out oversizing the ID of the block.

Now measure from the back edge of the block to the center of the gas passage, then compare that measurement from the barrel shoulder to center of the gas port on the barrel.  So block are produced for spacing for a front hand guard clip, while others are not.  Here what you are trying to figure out is if you need the block tight against the barrel shoulder or off set the distance for a front hand guard clip instead.

Now to install the gas block, make a mark on the barrel shoulder in line with the barrel gas port, and use the center line of the gas tube coming out from the gas block for your witness mark of where the block needs mounted to be on the barrel to index the block passage to gas port correctly.  So with the set screws in about a wind or two to start with, drop of Red loctite on the threads, install the block with gas tube over barrel shoulder mark, and tighten the set screws.
Note, if you need to remove the block later, insert a old allen wrench in the set screw, head the allen wrench up red hot with a torch, and when you smell a sweet smell, the loctite bond has broken down, and you can remove the set screws with a new allen wrench instead.

Gas block and tube installed. Pull the bolt off the carrier ,and using just the carrier with key in the upper receiver, adjust the gas tube over the barrel to index it correctly with the carrier key.

Considering that the gas block passage is much larger than the barrel gas port, who is to say the the pin once snapped in half is going to free drop into the bore (may bind on a bur), Hence the measuring and witness mark on the barrel shoulder cost nothing to start with.