The 1-10 will probably shoot those bullets fine. It really depends on the quality of the bore finish. If it is a top end barrel I would be suprised to see any bullet problems associated with twist.
If you were building a rifle specifically to shoot the 70 gr bullets, I would recommend a 1-12 twist but the 1-10 should perform well if in a quality barrel.
If there are problems with the 70 gr Sierra, all one needs to do is switch to the 70 gr Ballistic Tip and never have a worry again.
As far as powder, I would start with Rl-19 or a similiar burn rate Hodgdon powder such as H-4831SC. This powder as produced the best combo of velocity and accuracy in my rifles. They are all different so you could try any powder a bit faster or slower then this range.
I have not tested the VV powders in the 6-284 so I can not give any opinion on this.
Good Shooting!!
Kirby Allen(50)