Batch renaming and numbering song titles

Hello,
I have ripped an audio book from 6 CDs. Currently the tracks are named similarly for every cd: Track 1, Track 2, and so on. I would need to change the numbering to that the tracks would be from "Track 1" to "Track 70". Is it possible to do that with Mp3tag? So far I only managed to batch rename the tracks as "Track". (If not possible with Mp3tag, can anyone suggest some other way to do this?)
Thanks for help!

Sort the files in the preferred order.
Select all files
Use Converter>Tag-Filename
Format string: Track $num(%_counter%,2)