Drew, until you tool up you are going to have a rough ride.
Most of item on the rifle have been loctited from the factory.
On the butt stock screw, you just need a larger flat head screw driver to back the screw out (one that fills the slot). If you try to use heat, you end up cooking the butt stock butt plate.
From there, you will need to heat the old receiver extension just behind the receiver to break the bond. Once broken, you can use the flats on the back of the tube to spin it out. As it start to spin, reach inside the receiver and retain the buffer detent pin as it will pop out once the lip is spun back.
When you install the carbine receiver extension, you need to spin it in until slips over the buffer detent flat, then touches the center of the post. From there, its just a matter of slightly backing it out to index and the castle nut tightened down to hold the index. The end of extension lip needs to retain the buffer retaining detent.