The SST39VF040 is listed as untested on the flashrom supported hardware page.
I tried it on a 3Com 3C905C (which limits to 128kbytes). All operations (probe, read, erase and write were done successfully for the first 128kbytes) [0].
I attached the NIC's bootrom [1], which was resized from 64kbytes to 512kbytes in order to be flashed. I also attached the same image which has been read after the flash. All the first 128kbytes are identical, and the data repeats until the end of the file.
The network card is also able to boot from this eeprom.
Thanks,
[0]: http://monorailc.at/tmp/3c905c.tar.gz [1]: http://monorailc.at/tmp/3c905c_dump.tar.gz