If you can't use a jig, then you can try this.
Get some clamps.
Clamp the receiver solidly to the bench or in a vise. Use a machinists level on the bottom of the receiver to make sure it is level.
Clamp the mount base to the front receiver ring. Check to see if one hole, if drilled too deep, will go into the chamber. If so, make note of that.
Put the machinist's level crossways on the top of the mount. Move the mount base until it is perfectly level. Center punch through one of the base holes, lightly. Double check everything, including using a long straight edge in place of the level. Triple check. Twice.
Then center punch the other base hole. Double check. If all looks good, including by eyeball (it is surprising how accurate a trained eyeball can be), center punch normally and drill ONE hole in the drill press. MAKE SURE you don't drill too deep and drill into the chamber.
Screw down the front base with one screw and mount the scope with just the front base. Set the rear base in place and try the scope. (You are checking for mismatched bases before going any further.) Double check, etc.
If all is OK, remove the scope and the front base, and drill the other front base hole. Double check. Again. If all is OK, repeat the process with the rear base.
Make sure the bolt works; if not, you might have drilled into the bolt lug area behind the chamber and the screw is blocking the bolt from turning, or a rear base screw might be keeping the bolt from going into place. If so, remove the offending screw and shorten it until the bolt clears it. (Do not grind off the locking lug to clear the screw, as I heard of one fellow doing.)