Just for grins, set the bases and rings on...check to see if bolt handle has been altered to facilitate scope mounting (also safety). If the screws you are trying to use are 6X48, Brownells sells a slightly oversized screw and tap, don't have to redrill bases that way, 8X40 are the next step up.
If the bore of this gun is ok, might make a good project gun, slow down, identify the steps (and $) to put gun back in service and settle down to a nice winter of research and developement.