Sounds good, if it was a simple engine replacement, then the wiring should not have been molested or modified in any way by the previous owner.
A C2500 could have come factory with a 4.3, 5.0, or 5.7 gas engine.
The ECM would be the same for 1988 and 1989, however the PROM inside will be different, for the different engine calibrations.
So, you 'likely' have the correct throttle body ( injector flow rates are different ) on that 5.7, you have the correct computer ( oe # 1227747 ) but we have no idea if the original for the truck PROM was installed in the replacement computer.
My gut feeling is likely not, however, I can't read or burn a PROM for you.
As far as the speedometer, I am not familiar with the circuit diagram, but I would start with verifying that the speed sensor is working, there has to be a test for that. A service manual would be beneficial in this case. Next would be the wiring from the VSS, disconnect both ends and do a continuity test.
In the early models, the VSSB or DRAC is internal to the cluster, so it is possible you have a problem with a circuit board or solder joint inside after that many years.
This also means, that simply swapping the cluster means you need to find one with the correct axle ratio and tire size in the vssb located in the cluster.
GM # 25085515 recalibration kit is no longer available.