McMillan will make a stock for you in left hand, as for wheter it will allow 4 rounds to fit the box with the 404 case I cannot say, I am not sure that the standard model 70 box will hold 4 rounds in any event, based on the .458 Safari grade on the .532 head diameter I think the box only held 3 rounds down.

You may well need to go to a magnum magazine box such as those made by Blackburn (assuming you can make contact with him and get a delivery date this millenium) or even Sunny Hill will make a drop box for the model 70 that should hold 4 rounds, if you did this it would then be worth your while contacting McMillan and asking what they can do for you stock wise and for inletting to your bottom metal.