Actually it's really not that complicated on an AR.
The barrel will only fit (index) on one way due to the pin on the barrel extension, you really can't put it on wrong.
And as for headspacing on an AR, also very simple, as long as it's a new bolt and barrel combo they will headspace just fine.
The problem lies with matching up an older barrel with new bolt (or visa versa) since the locking lugs will be worn more on the older part.
If the sights are canted the most likely reason is the front sight/gas block taper pin holes were not drilled a true 90 degrees to top dead center.

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Hold my beer and watch this!