Depending on the quality of the tape, you'll be best off using VBR (variable bitrate) 'cause analog/tape sources require a lot of bitrate in converison as the panning of the tape rolling unevenly over the tape head causes the mp3 codec a lot of grief.
Look for a program called Razorlame, that's what I use. Soundforge will make MP3s for you, but not VBR (last I checked). You'd have to set the bitrate to 256kbps just to keep the flanging to a minimum, and this will create really large mp3 files. VBR will reduce the bitrate when it's not needed (quieter passages) and raise it when needed (louder passages, reduces the flanging).
BTW, once you have wave files created of your tapes, it's best to just burn them to cd (no compression) rather than converting/compressing them to mp3 only to be converted/decompressed back to waves when put on the cd... unless you're making MP3 cds (with several albums on them), you really don't need to even bother with MP3.