The answer to your question totally depends on the condition of the particular receiver that you bought.
The mauser 98 action, in general, is among one of the strongest designs in the world. However, consider the following factors:
1. The K-98, as with almost all military mausers, were not made of high grade nickle steel (as are current guns). They were made with carbon steel, which was generally okay for the type of round that they were designed to shoot. These receivers, however, are now more than 60 years old.
2. The K-98 was designed and manufactured specifically to shoot the 8mm Mauser, which has a chamber pressure in the area of 50,000 psi. Check the manuals on the two rounds you are considering. They may, even in factory loads, generate standard pressures of 65,000 psi or greater! The .308 Winchester, and .243 Winchester, are both in this category. Since the .358 Winchester is just a necked-up .308 Winchester, I imagine that it would be in this neighborhood as well.
3. Many many K-98s made during heavy wartime (1942 to 1945) were made by slave labor, in factories that were being bombed. When and where was yours made??
4. The best K-98s were made pre-war, during the years of 1938, 1939, 1940. Even these, however, could be stressed out or unsuitable due to abuse, rust, etc.
5. If you have a high quality pre-war K-98, then the last thing you want to do is to destroy its collectors value by re-barreling it.
The only way to answer your question is to have a qualified gunsmith, who is very experienced in custom mausers, examine your action. I would also suggest that you have it tested by the magna-flux method, to make sure that there are no small cracks.
If your receiver is covered with rust or pitted, then I would not go to the considerable time, trouble or risk of rebarrelling it.
The short answer is 'yes'. The m98 is fully capable of handling any standard and many magnum calibers(there are some length considerations). Considering the costs involved in making a custom rifle and the millions of mauser rifles made I doubt using one to create a custom rifle would be too expensive or that it would create a gap in the historical record.. The M98 action IS made of carbon steel and casehardened. In it's day mauser marketed an extensive line of sporting rifles and in nearly any imaginable caliber. Some british makers such aas Rigby chambered mauser made actions in some very powerful rounds indeed. The case hardened construction may and I reinterate MAY affect the ultimate yield strength of the action but I've not seen a failure yet..though I have seen ruined actions. The usual problem with a mauser fed a high pressure load is called 'imprinting'. It ruins the action but I've not seen a failure yet. I have seen maodern actions fail catastrophically but not the m98. Please understand that it's likely you will NOT be able to sell the rifle for what you'll have spent when it's finished but that's not unique to the mauser action, just the custom rifle. A M98 in 35 Whelen or 270 Winchester is a clasic and thousands have been made and many are still in use.. As an aside one should note here that the M98 is one of the finest actions extant should a CASE fail or a primer pierce. They are noted as MUCH better at handling the escaping high pressure gas than nearly all modern actions, including the M70 and M700..
A forum community dedicated to the great outdoors and hunting enthusiasts. Come join the discussion about hunting, fishing, survival, archery gunsmithing, optics, reviews, accessories, classifieds, and more!