Truck will not start

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Have you checked to see what is missing, spark or fuel? Here is a link that will help guide you through the condition you have. Since you replaced the ECM and now it won't start at all, I would recheck all connections. Also, before you unplug or plug in the ECM, the battery must be disconnected.

Take a look through this link and let me know if anything in it helps. Also, let me know if you checked fuel pressure and for ignition spark.

https://www.2carpros.com/articles/car-cranks-but-wont-start

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

https://www.2carpros.com/articles/how-to-test-an-ignition-system

Here are the directions specific to this vehicle for ECM replacement. See if you missed any of the steps.

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1989 Chevy Truck Suburban 1/2 Ton 4WD V8-350 5.7L
Engine Control Module (ECM) Replacement
Vehicle Powertrain Management Relays and Modules - Powertrain Management Relays and Modules - Computers and Control Systems Engine Control Module Service and Repair Procedures Engine Control Module (ECM) Replacement
ENGINE CONTROL MODULE (ECM) REPLACEMENT
When replacing a production ECM with a service ECM, transfer the broadcast code and production number from the production unit to the service unit. Also, during replacement, the PROM, and if equipped, the CALPAK must be removed from the ECM being replaced and transferred to the new unit. Refer to "PROM, REPLACE" and "CALPAK, REPLACE" for procedure.

1. Disconnect battery ground cable.
2. Remove right side kick panel, then disconnect two ECM electrical connectors.
3. Remove ECM mounting hardware.
4. Remove ECM from passenger compartment.
5. Reverse procedure to install.

Let me know what you find.

Joe