If I were to take a MCX rifle and install a brace and 9” barrel, would that be in compliance or does the lower have to be marked ‘pistol’? Pretty sure I’m GTG but confirming.

If the serial numbered receiver came from the factory built as a rifle, with a stock and 16" or more barrel (or was first built into that configuration from a virgin receiver), Then that receiver can NEVER be used to make a "pistol" configuration.  Legally it would be considered an SBR and subject to NFA regulation if you shortened the barrel to under 16", even if you removed the stock first.

You need a receiver that either came from the manufacturer on a complete "pistol" configured build, or you need to take a virgin receiver and first build it into a pistol configuration.  In neither case does the receiver need to be marked in any way as a pistol.

Copy....and thanks.