Dmax- a krieger barrel or equivalent is the fastest and cheapest way to an accurate rifle. the problem is it take 18 weeks to get the barrel. you could try a brux, broughton hart ect . i say get the the remington start shooting now.
The SAAMI spec cartridge length of the 300 Win Mag is 3.34". Winchester has historically manufactured the model 70 in 300 Win mag with a magazine to fit that length but not much extra. The Rem 700 uses their standard "long action" for the 300 win mag which has a 3.6" magazine.
I have a 300 Win mag Larado model which has good accuracy with factory ammo. I also have a Rem 700 Sendero, which can shoot heavier VLD bullets with handloads with up to a 3.60 cartridge overall length. That allows both more usable case capacity and higher ballistic coefficients. That results in longer range, more delivered energy, and lower wind deflection.
So my choice is ... both.