The 98 Mauser action is more than strong enough for the 35 Whelan. Your gunsmith should be able to tell you if the action is sound; that is, not cracked or bent and is safe to rebarrel.
The Turkish 8s sometimes had different barrel threads and I think some had a shorter barrel sleeve that took 96 Mauser threads, or some such. The thread size is something the gunsmith will have to determine so that when he orders a 35 Whelan barrel for that action he gets the right one. HTH. Mikey.