After working on my T-56 swap, and talking to Jay, I think I have a solution for this issue. I currently have my speedo wired this way and it is working. Here is a brief explanation of how the VSS works in the TBSS. The raw AC voltage from the VSS goes to the tcm, the tcm compares that voltage with the iss signal, and then sends a buffered signal out to the ecm, which in turn generates a 4000 pulse per mile signal and sends that out to your IPC (speedo). Here is my ghetto, hand drawn version of that.
I think the the reason for the 0 speedo at launch is because with a stall, it throws the ISS signal out of range of what the TCM is expecting based on the VSS signal it is receiving, so it defaults to 0.
What you need to do is install a 2nd ecm (E67), loaded with a tune from an 07 CTS-V which uses a manual trans. (I have both HPT and EFILive versions and can send them out upon request.) The CTS-V tune is designed to accept the raw AC voltage straight from the VSS, since there is no TCM or ISS, and generates the 4000 pulse per mile signal and sends that out to you IPC. The only change you will need to make to the CTS-V tune is to copy your tire size, pulse per mile, and gear ratio settings over from your original tune. Here are the settings for STOCK wheels, tires, and rear gears.
Basically all you need to do is run power and ground to the 2nd ECM, split both raw AC voltage wires coming off the VSS between the two ECMs (this can be done very easily just before the wires go into the TCM), and disconnect the wire that comes out of the TBSS ECM and goes to the IPC and connect it into the same spot on the 2nd ECM connector. What this does is, sends the raw AC voltage to both the 2nd ECM and the TCM. This way the TCM will still know how fast the vehicle is going. The TCM will still send the buffered signal to the TBSS ECM (when the ISS and VSS are in the correct range). The TBSS ECM will no longer be connected to the IPC. That wire will be removed from the TBSS ECM connector and installed into the 2nd ECM connector. Now the 2nd ECM is sending out the 4000 pulse per mile signal out to the IPC your speedo is working. Here is yet another ghetto, hand drawn schematic. (Some of the power feeds into the 2nd ECM may not be necessary but I wasnt sure so I listed them all)
The E67 ECM connectors can be purchased
here and come with all the pins as well.