Cutting the barrel back a couple of inches is not a bad idea, but I would not look at a slower twist barrel, the 1 in 8 will shoot lighter bulets. It will shoot 65gr match bullets all the way up to the 108's, and maybe the 115's. Varget is one of the most popular powders for a 6BR, second would be Reloader 15. Primers I find that the Remington 7 1/2 is about the best choice for a BR case, regardless of caliber, Most people shoot the CCi 450 Magnum primer, and it also works well.
I did a review a few years back of this rifle, you can access it from the main page of this site.