Good gravy Bil, but you're using a lot of bandwidth. Phew. I felt like I was at the drive-in movies. I turned the speakers on hoping to hear "Ride of the Valkyries". No luck.
But it's looking good. Great pictures and great detail. I made a cradle to hold the recoil booster, aka flash hider, from a piece of angle iron. I welded a section of all-thread to the bottom. With the nuts on, I could adjust it up and down. But it would have been better if the angle iron would remain fixed while the elevation was adjusted. I never got to that point. Instead I used js412000"s jig. I was intending to use a flexible clamp to hold the recoil booster in place once I had it positioned correctly. Somewhere there is a picture of it. Unfortunately I don't know how to link to it, but it is there.
Just remember to only tack weld and not try to completely weld the pieces together all at once.
I used the same set-up to hold the rear section in place like you are doing. It worked great.
At the cam holes, I used a copper backer and flat stock to keep everything aligned while I tack welded that section. Same with the nose section. From what I can see, you're doing fine. Just remember to take a deep breath and say a prayer to the receiver gods right before you strike that first arc on that irreplaceable receiver.