Do not use a barrel block when torqueing the barrel nut. That is the "old" way of doing it. And if anything, you can damage the index slot using that method. When using this method the receiver will want to "torque" in the direction of tightening.
However, use the barrel block to install the flash hider/muzzle brake instead of the receiver block. This will keep the barrel from moving during installation and won't have any effect on the receiver. The barrel block is positioned on the rear portion of the barrel, not the front (exposed) portion.
Use a receiver block for installing the barrel. This way provides less "torque" on the receiver (which is softer).
Also, I use a sight indexing rod to ensure correct alignment with the receiver and the barrel. This is a precision machined rod that lays on top of the carrying handle. The pointed end aligns with the front sight. As long as the pointed end of the rod stays in alignment with the front sight during barrel instrallation you will have perfect receiver to front sight alignment.