+1... YES... I have necked down 300 WSM for my 270 WSM. Perfect fit without any trimming. Slightly heavier neck but no problem for a no-turn-neck chamber.
You'll hear about twice as many good reports as bad about 6.5 WSM. I have thought about doing it many times. But I can get 3200 fps with 150gr AB and almost 3300 fps with 135-140gr with my 270 WSM. The 6.5 WSM can't.
Still, it would be a fun caliber.