start with that ^
then trace the wire from the speed sensor to the plug to make sure it is actually connected.
from there, I would say check the connection on the back of the cluster, it might be crappy or loose, or whatever from when the cluster was swapped.
Then, might have the change the cluster itself... they do go bad...

Have you been able to get fast enough to see if you're hitting the speed cut yet? That would narrow it down to being the cluster if the ECU is getting the signal.