Thread size varies from 4-48 on older Topper barrels to 3-48 and 6-48 on modern barrels, Williams makes several ramps that might work, the shorty ramp is a good match for my 30-30AI Handi with a 3-48 hole, you can either have it drilled and tapped 6-48 which is what the ramp comes with or just use the 3-48 screw from the original sight if you have it, which I've done on both the 44mag and 45-70.
Tim