Per JP's instructions, they advise securing the end piece( as you called it) to the reciever with red lock tight. I believe they have instructions on PDF through their site. I used the red lock tight method, I secured my upper receiver in the dpms claw (delrin block) made sure it was level. then took a bubble level peice and set it on the large hole on top of the end piece(as you called it) making sure it was level while the lock tight set. I went to lowes and purchased a string level and busted it out of the plastic piece you hang on the string. When assembling my rifle I ended up having to grind down one of the tangs as suggested above, just take your time. I will post some pics if you need clarification of the end peice w/ bubble level. There are other methods available, especially if you plan on adding rail sections for back up sights. Mine came out really good, but I don't ever plan on using flip/fixed sights attached to the hand guard.