"...all mauser actions standard length?..." Nope. Close but not exactly.
Like jmr40 says, stocks are rarely drop in. Even the one's that are sold as such.
Anyway, Gunparts lists 2 stocks for Interarms Mk X's. Inletted for an 8-5/8" long trigger guard and no mention of chambering. $144.60.
Looked at Boyd's' site but they're having site issues. A laminate might be a better idea strength wise.