A majority of machine work has now been completed to the magazine cavity and feed well but the rail width modification is still required. The OD of the belted case as well as the shoulder OD is larger than the 270/30-06 class of cases so additional room through the rails must be made to accommodate for these larger case diameters.
It is the height of the case that protrudes above the rails and allows the lower edge of the bolt face to engage the protruding case and then allow the bolt to shove it towards the chamber. The case must stick up through the rails ENOUGH as its pushed forward or the bolt face will override the case head and cause a jam. Most 270/30-06 Model actions are approximately .545 to .550 in width from one feed lip or rail to the other side at the very rear of the feed well opening directly above the magazine box and then widens to .600 to .610 directly above the bottom of the bullet ramp. This geometry has work since the 1st Pre-64 stepped off the bus so it's hard to argue that it isn't adequate, is it prefect? well...............
All the next cuts will be made from the center-line of the receiver so you must establish the best means to do so. Again the fixture I use allows me to do this very easily. Finding the center-line I start off and finish using only a 3/32" carbide end mill. I widen the existing width at the rear of the rail from the original .545 to .610. Taking a photo of this operation is difficult at best due to the close proximity of the mills spindle being so close underside of the action.
With the help of the DRO I will now cut stepped flats into the rails in incremental lengths from the rear forward and later on file and blend these transitions. If you had a CNC you could write a program to do this parabola shape, push start and step back.
To confuse the issue even more below is a standard Pre-64 action and OEM follower that is chambered for 280 Remington that has the ejection port opened up the 3.416. Note the tail of the extractor showing with the bolt in full closed battery and then the tail tucked under the rear bridge as the bolt is opened. The result is a longer loading port and a captured extractor tail.
The wonder of it all !
A lot depends in any conversion or modification on the the materials at ones disposal. You will see similar adaptations of this on many of the Magnum length actions chambered for long cartridges. Note the visible exposed tail on this extractor fit to this H&W magnum 98 action, lift the bolt and the tail end slips under the rear bridge before the bolt rotates completely up the extraction cam, no fuss-no muss.