Converting an Academy B-17G to the earlier tail turret is easy. Just steal one out of an Academy B-17E or F kit. Of course then you have the problem of what to do with that kit.
Seriously, I have tried, so far without good success, to make clear resin copies of the Academy G & E/F tail turrets. I was hoping to be able to transplant the Academy G turret to the Hasegawa Gs that I have still waiting to be built. The E/F tail turret would, as you point out, make earlier G models possible.