There is more to this than just thinking that if the radius matches, such and such bases will fit. One also has to take into account the requirement that the bases, and by extension the bottom of the rings, are level with one another.
Often if a receiver sight was used, the 'smith did not attempt to keep a standard difference between the rear bridge elevation and that of the receiver ring. In these instances, unless the rear is too high and can be ground down, the best option is custom bases. If this is not an option due to cost, shimming is next. However, one needs to make sure the shim is stable.
