When I built my XP-100 in 6.5mm WSM, I just used near top loads for the 6.5 Rem Mag as my starting loads and they worked great.
I can not recommend using 270 data even with 5% reduction in loads. Your 6.5 WSM will likely have a much tighter throat and chamber then any 270 so even starting data could be a little on the warm side.
Since the 6.5 Rem Mag is smaller then the 6.5 WSM by a fair amount, these result in great starting load levels.
Good Shooting!!
Kirby Allen(50)