DANTEC,
For an all out accuracy load and not worrying about velocity, I've always had very good luck with IMR7828 in the 6.5/300 WWH w/140gr class bullets. But the velocity is down near a good stiff 6.5-06AI or 6.5 Gibb load also. So your kind've wasting a 6.5-300 WWH by doing it. Just thought I would mention it based on your post's subject line.
I've tried it in 2 of my 6.5s and 2 others rifles owned by friends in HG and sporter configuration and IMR7828 improved accuracy in all of them with the slower speed. You will run into pressure before you match the speed of H870 or the surplus powders that Darryl pointed out.
When I hunt with mine 7828 stays home. Been to long though. Plus I haven't bought any new 7828 in many years so the burn rate may have changed alot sinse then also.
We usually ended up around 71 to 72gr of IMR7828 for accuracy. Remember to start low. Sorry had to say it.
Steve