It's not just a simple rechamber. It's a rebore and rechamber. In such cases it's really best to start with a chamber some what smaller than it will end up. Otherwise the smith will likely need to set it back a thread or two and that might result in an improperly bedded rifle. A rebarrel job is the easiest way to go if the rifle is to be converted.Can't you find a smith to rechamber the existing barrel? The .35 Whelen is a .30-06 necked up to the bigger bullet.