Re: 300 rum- 338 rum conversion
Never done the conversion myself. Not much to it other than a barrel and the labor to spin it on. At that point you may want to have some other work done. If it hasn't been done yet I would suggest at least a trigger job and bedding the action. Both are relatively inexpensive to do. You may also want to look into a muzzle brake.
I do own a rifle in a 338 rum and love the cartridge. My gun was built by Defensive Edge and has one of their brakes on it. The brake makes it a real pleasure to shoot.
Good luck and Welcome to LRH