The 6.5/06 is a performer all right, but also a bit of a barrel-burner too. Mine lost its guilt-edged accuracy at 1000 rounds. It shot .3MOA when fresh, .... opened up to 3/4MOA at 1200 rounds.
If your 25/06 brass chambers without much headspace, then opening the necks and fireforming with full loads makes sense. If some headspace is in fact present, that brass will still shoot well at first firing, if you expand the neck to 7mm, then carefully size down to create a false shoulder at zero headspace(slight resistance to bolt close). Or, ... you could drop your powder charge a grain or two and jam the bullet into the lands, this, to hold the casehead hard on the boltface.
Cream of wheat fireforming concentrates all the stretch at the case's web/body intersection, not the best way to do it, IMO.
In my rifle, new Lapua 30/06 brass necked down with a false shoulder shot just as well as the subsequent reloads with fully fireformed cases.
Being as you have a custom chamber, attention to neck clearance should be considered.
A .25 cal. case necked up will result in a thinner neck(hence more clearance), than a .30 cal. case necked down has.
In my case, accuracy improved with the thicker /06 necks ... they have 3 thou. clearance vs. double that from cases based on 25/06.
Having too little neck clearance for easy bullet release though, will dramatically increase pressures.
Mine likes 52 grains of Reloder 22, the 139gr Scenar, 10 thou. jump, at 2950fps.
.270 and .280 brass works well too, the shoulders carefully pushed back to zero headspace, then trimmed to length.
