The Wyattes model CFE-9 magazine and F-10 follower allows a Rem 700 Long action to be extended to handle up to 3.990" cartridge overall length. It rrequires minor machining of the action and bolt stop. but so does the shorter 3.825" Wyatt's magazine.
CFE-9 Magazine Box, Wyatt's Outdoor
The information I have is that the neck of a 7mm STW with the shank of a 180 Berger seated 0.278" deep (so the shank is at the shoulder) will give a cartridge overall length of 3.877" Personally I like to keep the shank of a bottail bullet a bit in front of the shoulder for good velocity consistancy. I would use the longer Wyatt's magazine if going to the effort to do the modificaitions.
Before modifying the action for an extended magazine you might want to test the rifle as a single shot to see if the barrel's throat works well with the seating depth and overall cartridge length you plan to use. . The modificaitons for an extended magazine don't change that. You can of course extend the throat depth with a throat reamer if necessary.
So far I haven't made the decision to shoot 180 Bergers regularly in my 7mm STW's. At ranges to 1000 yards lighter bulllets have some advantages, like better accuracy unless wind deflection is dominant.