What brass are you sizing down? Lapua?
If its for a standard 708 chamber you might not need to do anything, just size down/normal prep and shoot.
Measure the OD of a fired neck in the 708,
Make up a dummy round with your sized down brass and compare the OD with the bullet seated, and if your 1-2thou under the fired neck OD your good to go.
If not, then lightly turn down the neck on the brass before sizing it down.
I use sized down lapua 308 brass, and have no problems.. No donuts inside.. No need for reamers etc.
And I'm seating 162 amax to AICS mag length.
Try it before you spend any money on unnecessary tools!