Gas block passage to the barrel port alignment, and you have to use 272 loctite on the gas block bolts so they will loosen over and over again.

As for getting the alignment correct,  index mark on the TDC of the barrel shoulder with the gas port, then use the bottom of the tube in play to align over the mark.

Also, get the gas tube bent to align it carrier key with the bolt removed from the carrier during a dry fit of the block to barrel, and do not rotate the block on the barrel off the  shoulder marks to align the key to gas tube that way instead.

If you need to remove the gas block at a later date, the old alllen wrench in the block bolts, heat the wrench up with a torch until the wrench glows to transfer heat to the set screw to break the loctite bond (will smell a sweet smell when it the bond breaks down, the remove the hot wrench and use a new wrench to remove the set screws while they are still hot.

Also, don' go bulk wild with the 272. Start the set screw in about 3 winds, put a drop of loctite on the clean exposed threads of both, then tighten the set screws down the last the way.  Hence we want to loctite the set screws in place to the channel only, and not leach the 272 down the channel to loctite the block the barrel instead.

Short of that, check the key to carrier for a leak between the two.  Hence hold the bolt inward, put some CLP around the base of the key to carrier edges, then pressurized the front of the key with about 100lbs of air and make sure that the bottom of key is not leaking to the top of carrier.  If you find a leak, then just send the entire new B/C back for a replacement.