Be careful putting lots of torque on that with the vice block in the mag well, keep in mind that all of the torque applied will be transmitted to the lower through the two pins that mate the upper and lower, as well as the metal they're embedded in. There are clamshell type vice blocks that allow you to put the upper in a vice by itself, which eliminates any possible bending of the upper/lower pin mating points.
Example:
https://www.wheelertools.com/gunsmit...4.html#start=1