I had to make my own, about 20 years ago, in order to mount a first-generation Aimpoint on my Ruger Mark I (tang safety) .308 M77RSI.
If you have a dealer near you, who has a box full od Weaver one-piece bases to choose from, paw through the bases until you find the cantilevered one for (IIRC) a Marlin 39 - anyway, the base you want will have a thin section (for over the front Ruger scope mount dovetail) and a thicker rear (for over the lower, rear Ruger dovetail).
Measure the height difference between your front and rear receiver scope mount dovetail flats (not the scope ring tops), and machine the thick part of the Weaver base bottom to meet that spec - to match an existing hole location in the issue Weaver base.
D/T one screw in each Ruger Reciever scope dovetail flat - hopefully in the center, for the altered base.
One screw held my Aimpoint w/o issues for several years, but there's no reason extra screws cannot be installed if desired.
