Could 280 brass be used to neck down to fire form in a 6.5-06 Ackley?

Would it be the best brass or 270 brass?

Not interested in necking up 25-06.

Either 280 or 270 brass could be necked down to fireform into a 6.5-06 Ackley. Just adjust the die to the point where you get a "crush fit" when chambering the round which will eliminate any headspace issues you might have with your chamber to prevent brass stretch above the base/web.
You will have to trim the brass. You can fireform using bullets or COW.

Barstooler

Hey Barstooler, thanks! was what I was thinking but wanted to confirm.