Not to sound like bubba....but, nobody wants your old barrel nut , unless you want to keep it. I had the
same problem on my LAR 8. I had to use a pipe wrench to get the original nut off. I was able to use a
standard ar15 combi wrench to install the quad rail's barrel nut. I used the one that looks about a foot long,
looks like a big flat blade screwdriver on one end . This type typically does not have a torque wrench provision,
but at a starting point of 30 ft/lbs. you should be able to "feel" it. On mine I went to one notch in the barrel nut
that felt right, tried to go to the next and couldn't. I backed it off completely and went back to the first lined up
notch. One other point, the wrench I am using ( and probably most ar15 types) won't grab the nut 180 degrees apart,
like a 12 & 6 o'clock position. It was more like 1 & 5 o'clock, so care must be taken to avoid slippage.I had mine
apart several times during different stages of the build with no issues , using this technique.
ETA: The wrench from dpms in your link looks just like mine.