Stick sometimes goes Yoda on us....
I'm ignorant of both calibers, but have done a couple conversions elsewhere - .270 and 06 to belted mags. My 700 is 600 miles away and not used and disassembled in years....tho I do have my 725 SA here. Excepting the trigger mech, they are mostly the same - in fact it is wearing a 700 takeoff-replacement barrel that screwed right on perfectly with no other gunsmithing- although it IS exactly upside down...
First is mag box length, which I am going to assume is compatible. Also mag box width/binding, sometimes a result of glass-bedding the inlet or an improper inlet. Mag box should have stock clearance all around, and preferably just a slightly loose (1/32 to 1/16) fit between receiver and floor plate with receiver screws fully tightened. If it worked in original stock/caliber and still in same stock, probably not the problem.
Make sure the mag on the 700 is seating into the recvr and floor-plate properly when assembled.
Cycle it slow, visually and see what you see...
Unless cartridge diameters are wildly different I'm guessing the follower is not the problem.
Overriding is usually caused by a weak or mis-installed follower spring, magazine binding, or rail-width problems. Or some combination.
The spring needs to be positioned so it pushes the BASE of the cartridge up where the bolt face can catch it. Some springs on some guns (can't remember on the 700, offhand) can be installed upside down, and the bolt then overrides the cartridge. Try reversing the spring if it is so constructed. Just checked the 725 - the mag spring has a short leg and a longer leg don't remember if this a replacement 700 spring or not, but the long leg needs to go into the follower, short leg into floor plate.
If the cartridge feeds with less than a full magazine, it.s likely the follower spring is weak and needs replacing.
If the cartridge stack in a "full" magazine in the rifle does not have a little play in it when pushed down, the cartridges may be jamming against the rails, and the magazine may need just a tad more vertical relief as stated above- or use one less cartridge.
