I have a similar plan to transform a tikka into a 6.5 PRC - I picked up a 270WSM, modified the bolt stop, and also customized the magazine for bullet length(was a fun challenge) I can keep using it as 270WSM until I trade the barrel. Another forum user "284saum" used a 338WM and didn't have to modify anything, although he did put a new stock on it. Pretty sweet looking BTW. He had a smith install the new barrel and has posted some awesome results. Check out the post - 6.5 PRC Tikka custom -
You might consider keeping that 270 wsm and putting a 1:8 twist barrel on it. You can shoot 145 eld-x, 156 hammer hunters, 165 matrix and 170 berger eol bullets all at over 3000 fps. Makes for one heck of a deer and elk rifle and is no slouch for long range shooting as well.