you can do every part of the barrel and the build except chamber it right now.Gharr! No use working on the barrel until I get it chambered. Can't chamber without a reamer. Can't order the reamer until I get some brass. Which has been back-ordered for six weeks already...
you are going to need to determine bolt clearance and extractor groove clearance before you can cut the chamber. the way I do mine is cut the extractor groove and turn the the shank (in your case turn the shank and cut the threads ) and set the barrel in the trunion at the correct depth to give the proper bolt to breach clearance. once I know thats right I chamber it with the trunion on the barrel. I have done it seprate of the trunion but it ads a few more steps I also have short chamberd them and then hand reamed it to final head space depth as well.
you already know how thick the shank and thread size needs to be you also can now determine the dia of the barrel were the RSB,GB, hand guard retainers if your using them, FSB, ect as well as all the work for the brake can be done. 95% of the gun can be built with out the reamer. Id do it as a screw build leaving the holes the original diamiter so you can get all the bugs worked out with the feed ramp/bullet guide with out the reciver attached. if you go screw build initaly the entire gun minus the chambering can be built now and then the trunion can simply be unscrewed the barrel /trunion asssembly reamed for the chamber and reinstalled. once the bugs are worked out the barrel can be pulled and a rivet job can be performed.
keep in mind that with a screw in barrel build the rsb needs to be removed or at least slid back to un screw the barrel. on my varmit build stuff I press the barrrel from the breach end.
IMOO your barrel purchase dictates that your going to build some sort of 50 cal AK if the beowulf brass dont come through you can still have the gun built 99% and go with a 500 S&W or some other wild cat still