Order an EGW picitinny rail(0 MOA) for the military 98 action. Order it without the screw holes. It will be machined to clear the stripper clip hump on the rear bridge:
https://www.egwguns.com/catalogsearch/result/?q=Mauser+98Order a brownells reamer for 6-48 fillister or tapered screws. Whichever screws you prefer. If you go tapered reamer, you can use the supplied egw screws.
If you know what you're doing, you can center this rail while using the existing holes, which were probably drilled into the receiver by some idiot, without using a jig.
Snug down the front screws. If the rear of the rail floats and doesn't mate to the receiver bridge perfectly, shim it, then cinch down the rear bridge screws.
The one piece EGW rail without holes is your friend. It'll get you on paper if you do it right.
I envy guys with commercial fn receivers, which were drilled and taped properly from the factory with industry standard patterns.
I have fixed three mausers with these rails: a parker hale 243 winchester, a Santa Fe arms 30-06 and a Waffenfabrik Oberndorf Mauser. All with crooked scope base holes. The parker hale needed a shim on the rear bridge.
If you mess up, youre only out $40 for the rail. Try again.
Once you get the rigid rail true to the action, you can do some cutting of the rail and convert it to 2 piece bases If you like an unimpeded action area for loading of cartridges.
