I don't know about the Stag lower model, but they have the cover of the new Guns & Ammo of a lefty and right 9mm model. My question would be if the upper would be compatible with the 5.56 lower. Where the mag goes looks like it is a 5.56 opening.
Get yourself a CMMG MK9 Complete 9mm Lower for about $325.00 , it's a dedicated 9mm lower. No blocks to deal with.Then get a complete 9mm upper. A couple friends of mine got some theirs from JSE Surplus. Out of the box no issues with either one and they both shot awesome with c products mags, colt mags & modified uzi mags.