I am looking at starting a new build and need some help. The action I am looking at a friend of mine has as a complete rifle is a safari express in a 458 win. mag. What i would like to built out of it is a elr. The calibers I am looking at are 338 lapua improved or a 375 chevytac. Please tell me what changes needed for each caliber and any advise. I would like to have it as a repeater.