Yes, a 1991 needle cluster is somewhat tricky to find.
Are you 100% sure they sold you the correct item though? Flip it over; it should have a metal backing plate and a pin-style connector. If there's no metal plate and it has a "finger" style connector, you got hosed..
Next up, in the event it's the correct cluster - are you prepared to calibrate the speedometer to your axle ratio and tire size? -->
https://www.chevytalk.org/fusionbb/showtopic.php?tid/258562
Also, you'll need to move 3 wires around in your harness before plugging the cluster in; this is necessary when installing a 1991 cluster into a 1988-1990 truck. If you've got the info handy great; if not I'll have to do some hunting as I've posted it a few times but never seem to have it ready to go.
Final potential issue would be, hopefully the cluster came from a truck with same # of cylinders as yours, so that the tach reads correctly.
Richard