I personally do not like the idea of converting a receiver set up for a small round to a significantly fatter round. That is unless you use a centerfeed magazine box which solves alot of feeding issues. The WSM would be my main concern. they do not always feed well in rifles actually designed for them but in a receiver set up for this very small round, there can be even more problems. Not that it is not possible, it certainly is but there are much easier ways to skin a cat!!
Also, the two rounds you list are about the longest of the short action rounds. There is a reason that Remington came out with the SAUM rounds, they are shorter and fit the Rem 700 much better then the WSM.
If you plan on using the LR bullets that most on this site like to use, it is very difficult to set up a rifle to accept ammo as a repeater and still seat the bullets close to the lands. It generally takes an extremely short throat to do so, even with some of the stretched magazine boxes. As such there is a liability issue concerning the issue of factory ammo making its way into a very short throated rifle.
Unless you have the 6.8 SPC receiver, I would never recommend buying one just to convert it to a larger caliber. There are so many out there already set up properly for the WSM that only need accurizing which will save you a load of money in the end.
Just my opinion. If you have the SPC already, it is certainly possible but needs some special things to make it work well.
Good Shooting,
Kirby Allen(50)