Re: KAR 98 MAUSER build
Before starting your build, you should pay attention to the manufacture date of the rifle. The German mauser '98's made in 1943 and after are known to have suspect heat treating of the receivers and should be hardness tested before having any work done on them.
If you have determined that you have a buildable receiver, you will find a lot in the way of parts available for your mauser. Bell & Carlson, in particular, offers quite a variety of stocks for the '98 mausers. Suitable barrels are easy to find as well. I have had good luck with the pre-threaded and chambered barrels offered by MidwayUSA. There are other choices available too.
.300 Weatherby will be too long to fit in a mauser action, but a .300WM would work if you modify the bolt face, extractor, and feed rails.
Good luck with your build!