Here's a picture(hopefully) of what you're doin:
It's good that you've decided -up front, to turn the necks. This will be needed as forming down the WSM case will greatly thicken the brass. A WSSM case is basically a formed down WSM case. That's why the WSSM brass is ungodly thick, and a total failure in Winchester's form. Maybe they really don't know what they're doing..
Also, I cannot stress enough, that you must have the brass in-hand to properly fill out a reamer sheet. In the pic I added my 6.5WSSM, and you might notice it is actually a different diameter(.551) than it's shown in any available drawing. If I had chosen .557(based on .555) for my reamer, .200 from the base, I would not get 5 firings from my brass. I went .553 and have well over 20reloads so far.
I wonder if you would REALLY be able to use it in a short action. I suppose it depends on which action and the loading port length. I use a long action BAT with my 6.5WSSM, and there is not alot of extra room. Reason being, 140gr bullets a VERY long. You will end up 1/4" shorter than a 260. Is a 260 a short action cartridge?
There is also the action diameter to consider. WSM/WSSM factory actions are larger in diameter, and I went with a magnum BAT action. They need to be larger diameter to have enough BARREL STEEL around the chamber. Otherwise the brass will grow like no tomorrow.
You may or may not gain overall compared to going 6.5-284, as it will be alot of work.
My 6.5WSSM is not better than any 260..
On the otherhand, it's no worse, and your 6.5Slic will be a fine cartridge I'm sure.
Just a matter of commiting to a larger boltface..
Good luck to ya, & have fun with it.