What model M70 did you get. This would determine which contour I would recommend.
I would personally go with a Lilja 1-9 if you want to shoot the 240s. A 1-10 would be right for the 180-220 gr pills but since you want to try the 240s a 1-9 will be needed.
I would personally say stay away from the 240s. The reason is because the newer, lighter long range bullets will get you nearly the same BC but with a higher velocity potential and allow you to stay with the 1-10 twist but thats just my personal opinion.
Barrel length, if your going with a sporter rifle, 27" would be about max I would recommend. In a Lerado type rifle I would say you may be able to push 30" but the barrel shank on the M70 is pretty short and small in diameter so I may limit it to 28" just to be safe. That is plenty of barrel for a 300 RUM.
I would recommend a Defensive Edge brake. The diameter would depend on the barrel contour you go with. Matching the muzzle brake
diameter to the muzzle diameter may be an issue on the lighter contour barrels.
I would much rather have a slightly larger diameter brake and get the benefits of a partition style brake like the DE brake then get a brake to match the barrel diameter and be limited to the performance of a brake such as the Vias.
Just my opinion.