I don't know anybody that has ever been able to get close to the listed velocitys that Lazzeroni gets. Their ammo is very expensive ,their reloading componets are also very expensive and from what I've seen their brass is crap.
I'd personaly go with either a 7mmRUM in a tight chamber or a 7mm-338Lapua Imp either way you'll get better reloading componets and probably better velocity.
I was actualy kicking around the idea of a 7mm-338RUM for a while, I realy don't think that the velocity increase from the bigger 300RUM case would offer any advantage over the slightly shorter 338RUM case being that they are both badly overbore.
As for the barrel , Lilja is definatly one of the best and I'd say that 28" would be the minimum for one of these bad boys. I'm not sure what you plan to shoot with it as far as range and critters but step the twist rate up a bit to shoot either the 180gr Berger VLD's or the 200gr Wildcats