The Stevens Favorite is not a good candidate for conversion to the .25-20 or any centerfire cartridge. The Favorite is a small action with takedown which renders it unsuited for the higher pressure centerfires - the single-shot Stevens action which is suitable for the conversion is the #44, which is both larger overall and has a larger diameter threaded barrel shank. The best and strongest of the Stevens single shot actions is the #44 1/2, but those actions are fewer and more expensive. The #44 action would be suitable for the .25-20 in its standard velocity loadings, though the high-velocity loads should be avoided.
PRD1 - mhb - MIke