TRACK can't be changed. I is built into Mp3tag's web sources structure. You need a action to change this afterwards.
However, while experimenting with my script, I stumbled across something which I belive is a bug of Mp3tag:
/t/11831/1
You can experiment with it. Let me know if you get a stable solution to change TRACK.
Are you sure you want to write the discogs lenth into you mp3 file? Every mediaplayer can show you the actual length of your files. What is the worth of having a second length form discogs which will be a few seconds shorter or longer in most cases?
I could make a option for that. I just thought it would be nonsense.
EDIT:
I just see it's already possible with my DISCOGS~PONE COPY ... options.
Set the following options:
DISCOGS~PONE COPY 1 SOURCE: _length
DISCOGS~PONE COPY 1 TARGET: length
and export the scripts again (action: Discogs Pone#Create Scripts...)