In regards to the parent cases, i.e.: 380 and 30-06, the 280 has its shoulder moved forward .050. That was done as I understand to keep someone from clambering it in a 270.
So, with that in mind I m guessing a true 280 and true 30-06 that are Ackley improved in the normal manner would continue to have the difference in length to the shoulder of the parent cases. That's just a guess on my part. Besides that both of the cases would have the 40 degree shoulder angle and minimized case taper. Necking either of the improved cases to 6.5mm would provide near ballistic twins. That said, you can buy new 280 AI brass and merely run it through a 6.5 -280AI Die and be ready to shoot.