Remington manufacture three 700 models in 270WSM and 300WSM calbres, which would be the ideal basis for a custom re-barrelling. Check the link:
If you plan to use the mag box you will be limited to the size of bullet you can use with the factory WSM action.
You can go for a long action with a magnum boltface or you can use any long action and have bolt face opened out.
This will enable you to take advantage of all bullets lengths avalable.
If you take the above route, modifications will have be carried out to the action to ensure reliable feeding from the mag box.
"I meant to shoot the pike but the duck got in the way"