The 308 WIN case is the parent case of the 6 BR.
You need a set of conversion (forming) dies, annealing, possible neck turning, and lots of work.
With some of those specific forming dies out of production or as special order, you can go 30 BR, 7BR, trim, turn, etc. 6 BR.
We used to do this back in the stone age, but don't need to any more.
These are not my photos, but will give you an idea of the work involved, less showing you the annealing.
But why would you if you can just buy 6 BR?
I think you have also confused the relationship between 6 PPC and 7.62x39. The 7.62x39 doesn't convert to 6 BR.
I don't recommend this path. It is a lot of work to end up with something you can buy.
Save the effort for Wildcats you cannot buy. YMMV