Check for spark at dist. cap no spark ..check at coil. Check for power at coil.
No spark at coil probably the ignition control module ..can take it to auto parts for a test
Would not think ecm . Do you have a service engine light? Would get a code for bad ecm.
Here is a link that will help check things good for 7.0/454 also. just click on red link Part 1 -GM Misfire, No Spark/No Start Tests (4.3L, 5.0L, 5.7L).