HC I had a feeling you must have gotten a new book or two to look at, judging from all the talk as to new ideas on big bores. That book has a lot to take in, it sits by my keyboard as I type. I look at it on a daily basis
No I dont think that a shortend 308 die would work as 1.5" from the end would cause the base/rim to be to small of a OD due to the taper of the case. the 308 barnes has a shoulder diamiter of .450 and the 308 WIN has a shoulder dia of .488. the shoulder angles are differant as well There are dies avaliable I already have the trim die. I made up a couple un FL sized cases to look at. the case is bigger in OD but the neck is further back than a 7.62x39. it looks prety much like a benchrest case except it is 30 CAL it does hold around 4 grains more powder and is susposed to out perform the 30-30 win in every way some thing the 7.62x39 does not. there is a 1.75" version (next page) that may offer even more, but likely to long for a standard AK mag.
I was thinking the 308x1.5 necked up to 9mm would have worked good for your 9x39 project.