Tony,
Sad to say the Ruger magazine will not accept a cartridge over the industry standard 1.61 inches. I tried helping a fella a few years back modify his magazine and we had no luck.
As a replacement to the 300 grain XTP go for Marshall's 280 grain WFN GC. This should give no trouble feeding through the Ruger and will certainly do anything the XTP will. Work up a load using H110 or WW296 and no hog or deer will stand a chance.