I have a 1998 Vortec Black Box PCM here on the bench so I gave the Flash tool a test. 98-00 are same although they do have several OS. Looks to be working but seed/key issue? Here is what I got. If you'd like to add this just walk me through what is needed.

Init:
AVT device reset ok
Model: 852
Firmware: 1.1
Identify:
AVT device reset ok
AVT VPW mode ok
Set AVT filter
Received OSID: 16263425
Failed to read block 8
Received Calibration ID: 0
Failed to read block 7
Received PCM Serial No: QC724786
Received VIN: 2GCEC19RXW1123841
Received BCC: CCSF
Received MEC: 0
Disable message transmission accepted
Calculated Key: D4F6
PCM Failed to Unlock
Also with the first flash test software you sent I used the Identify PCM and got this.
TX: F1 A5
Avt device reset ok
TX: E1 33
Avt VPW mode ok
Set AVT filter
Successful read of block 10
Received OSID: 16263425
Failed to read block 8
Received PCMID: 0
Successful read of block 5
Successful read of block 6
Failed to read block 7
Successful read of block 7
Received PCM Serial No: QC724786
Successful read of block 1
Successful read of block 2
Successful read of block 3
Received VIN: 2GCEC19RXW1123841
Successful read of block 20
Received BCC: CCSF
Successful read of block 160
Received MEC: 0