In my 1988 C3500 5.7L TH400 after replacing my original cluster with a digital cluster, the speedometer is irratic and not accurate at all.
ALSO the digital fuel gauge is not working with the original wire from the sensor in the tank

 

Have you called the people who sold you the cluster, or the manufacturer? They are the ones with the correct answers.

 

It'll be good to know the manufacturer of your cluster. A lot of times you'll need to calibrate the speedo, and fuel gauge for them to work properly.