Here's the deal - all you need to do is torque the barrel nut on. That's assuming the front sight is already installed.
Everyone wants to do the right thing and see you finish it up properly. We can't really assess your mechanical skills and understanding of what could go wrong. With that in mind, if you mark up the upper or get it wrong, sorry. That's happened by others using the right tools. On the other hand, all that work to deliver a scratch free finish to the soldier is answered the first day practicing the three second rush with drop and roll. A combat rifle will get torn up, whats a few more scratches?
However you clamp the upper, you can tell if you need a torque wrench pretty quick. Tightening the barrel nut to 30 pounds just gets the burrs off the threads, and how much the serrations line up at that point tells you a lot. If they are 50% or better, it won't take much more to get where the gas tube fits. The whole point of the 80 pound setting is to prevent damage to the threads from overtightening. If it's going to be a problem, you have to take off material from the front of the upper where the barrel extension shoulder rests against it. Squaring that face does that anyway, especially to true it up to a 90* perpendicular to the bore/upper axis.
Point being if you can set it up and tighten the barrel nut to 30, you can quickly see how much further to go, and whether you need the torque wrench or not. Point being, you can use a pipe wrench and block of wood, but will you? That goes back to assessing skill levels. Only you can do that.