IIRC,A Warren Page designed wildcat is the 260 AAR,for "All Around Rifle"It is a257 Roberts Ackley Improved ,but a 6.5 instead of a .257.
I have seen reamers and dies listed in Midway,I believe,as "6.5x257 Ackley Improved",it is standard enough you won't have to spend a fortune.
You can neck down 7x57 easily to fireform.
For my time and effort,I absolutely agree the .308 family of cartridges ,including the .260,will likely have feed problems in a Mauser.To me,it just makes more sense to use the family of brass the rifle was designed around.
I would not look so much to the cartridge for accuracy.
A little blueprinting on the action,like lapping the lugs(don't just pull back on the bolt handle,there is a proper tool)
A squaring cut on the face of the receiver ring..
Fitting the bbl threads,and how the bbl seats to the receiver..A sharp reamer cutting a nice throat..
A good barrel
Bedded,strain free and free floated in a stiff,stable stock.
Good trigger,quality optics
brass prep,developing the right load,straight,concentric ammo and shooting well,all those things will matter more to accuracy than the cartridge name.
Good luck,sounds like you have a good plan,pretty much,except I also agree that if your Mauser is a complete,original rifle ....I'd discourage altering it.
Do as you wish with what is yours,but you will not save any money altering a milsurp.