I also have a shooter grade model 99 (clip feed model) that I am going to modify for elk hunting. I am going 358 win as the ballistics and permanent wound channel are even better than the 338 federal which I view as a sub caliber .358WIN with the neck size having gone the wrong way. Plus... much better choice of bullets with those bullets designed to open up reliably at the more sedate velocities of this class of cartridges.
Personal opinion and rant over...
As to the original question, there should be no problems whatsoever with the conversion to either cartridge. I will agree that either is a marked improvement over the .308 for Elk sized stuff, and way cooler to boot! Good luck and good hunting.
