The documentation for the tar option
--delete has this interesting text :
This operation will rewrite the archive. You can only use `--delete' on an archive if the archive device allows you to write to any point on the media, such as a disk; because of this, it does not work on magnetic tapes.
As this requires the media to support random reads/writes, this might with
a bit of luck mean that
--delete is done in-place without doing
unpack in order to create a a new instance of the archive.
In this case, you might unpack 20 GB of files and then delete them
in order to shrink the archive. Repeat the operation five (or six) times.
I would advise to unpack the files from the end of the archive
and delete them in reverse order of the archive. If tar is intelligent enough,
it will just shrink the archive each time the last file in the archive
is deleted, and it wouldn't this way need to copy all the following files
again and again for each delete.
If this doesn't work, your only option would be to find another disk to
contain the untarred files.