That should be doable although the longer 30-06 case will definitley want the magzine box extended a bit, as you already noted. The 35 Whelan is about the largest caliber for that case that will still allow a reasonable should to headspace on.. It seems a very efficient caliber that kills well... In the 98 that would make a very efficient killer for animals up to elk or so... The magazine extension would be the biggest problem with a small possibility that the feed rails will have to be worked on.. Since you already tried them they should work OK..