XMas Greetings from Oz! Sounds like an exciting project, and you have obviously thought about it, as you choosing quality tried and proven components for the rifle.
However, I would query the choice of a number 7 contour barrel, as they normally weigh about 5.5 pounds, and that would make your rifle about 10.5 - 11.0 pounds. That would not present a problem if used solely for a varmint rifle, but way too heavy if you suddenly need to carry it any distance through the scrub, and swing on to a moving feral pig or goat.
I currently have two rifles that each began their life as a 25/06,and were used in that mode for about 300 shots before I had my gunsmith set back the barrel and rechamber to the 25/06 AI.
The better of the two rifles, I built (in about 1980) on a Sako L61R action, and a Shilen number 3 contour sporter(.610 in at muzzle) 24 inch 1 in 10 twist barrel. After adding a Canjar set trigger, fajen walnut stock with Pachmyer decelerator pad, Sako mounts and a recently aquired Bushnell Elite 4200 4-16x40 scope the rifle ends up at about 4 kg or 8.5 lbs. Although it was designed to be a hunting rifle for use on medium game (roos, emus, goats, pigs, etc) the sub 0.5 moa accuracy with the Nosler 100 and 115 grain BT bullets, means that it could be used successfuly as a varmint rifle if the need arose.
The 24 inch barrel on this custom rifle achieved 3,325 fps with the 100 grain bullet as a 25/06, and 3,550 fps with the 25/06 AI. Also it reached 3,075 fps with the 115 grain bullet as a 25/06, and 3300 fps as a 25/06 AI.
Unless you have discovered a "Calici free" area of Oz, there probably are not enough rabbits to toast the throat of your 25/06. However, if you are lucky enough to be frequently firing 3 shots or more in less than 3 minutes, then I would consider using a much smaller case and calibre.
The advice of fifty driver (Kirby) in an earlier post as usual is excellent, and I would also suggest that if you use the 257 calibre, go with a 1 in 8 twist barrel as that would give you the flexibility to use the heavier 130 - 150 grain bullets now being produced by Wildcat bullets. (I have placed an order for some)
I have found that in both the 25/06 and AI, the Nosler 100 grain BT is dynamite on Aussie medium game. I have only just started to try the Nosler 115 grain BT, and although very accurate, it will take several years of use in varying situations before I feel qualified to make an assessment on this bullet.
I believe the Wildcat Bullets, bonded core 130 and 140 grain bullets should provide the extra penetration on the largest of the mud crusted boars you might encounter, and the 145 grain ULD would certainly be an asset in the wind past 250 yards. The advent of these bullets should make the 257 compete with the heavier 140 - 150 grain 264 bulets.
Good luck on your project, Brian.
