The Speedo, BCM & TCM need to have compatible OS's. I believe the Speedo gets its signal from the BCM, I had it a bit easier swapping a 6L90 because I was able to use a factory harness from a HD2500 that came with the 6L90.
And even if the connectors are the same it doesn't mean that the leads back to the BCM have the right connections. My guess is if you trace the wiring back to the BCM from the speedo you'll find the issue in the wiring or OS's.
Good Luck