"You can use the 270 die and try the 260 mandrel with it."
You could try it. Doubt it will wo't work tho. There is a fixed limit to how far down any collet can be squeezed. The "tube" is the lower end of the collet, it must be pushed up by the shell holder, with the case fully contained inside, for the upper collet end to work on the neck by compressing it against the sizing mandrel/decapper rods show in Wood's photos. Therefore, it's length dependant by case class and a .260 collet is too short to allow the tube end to touch the shell holder when a longer length case is inside the die.
Lee's collet neck sizer is the absolute best available for factory chambered rifles, IMHO. I haven't used a conventional or bushing neck die since I got my first Lee some 18 years ago. I suspect Lee's price for a custom neck die is less than the cost of RCBS' less effective neck dies for a wildcat. And I have three old RCBS neck dies I no longer use for just that reason (.22-250, .243, 6mm International).