The barrel will be a #3 contour. My question is will a large-ring 98 mauser stock fit without major issues? Or, do I need to find a stock specific to the Mark X?

In short, what stocks will fit it?

Any 98 Mauser stock will work. Stocks cut for the MK10 specifically have a cut for the safety button/lever and sometimes a longer mag opening. It is a 98. I have a semi Inletted Mannlicher stock for a MK10 if you are interested, PM me.

I have several patterns I can cut you. The MKX like dpcd said has a side safety and often a MKX stock has that cut. I happen to cut mainly what I would call a commercial. No safety and the front guard tang is shorter so it will fit a commercial, MKX and even military with a touch of cutting.

Often a M98 stock has the bottom inletting cut to fit a commercial bottom. Leaving a gap in front of the front guard tang.

So long answer is use a MKX or commercial inletting. Only use a M98 if you are sure the front guard tang is cut short.

By the way all MKX actions are the same OAL. The short 243 style has a spacer in the box. The 375 has an extension on the front of the box. All MKX will fit in any MKX stock regardless of original chambering.