I have a 1997 gmc with a 5.7 it was running fine then bang blew the muffler wide open now it won't start back up its got spark and gas timming is good changed the plugs and cap compression is good now what's left

You may have blown the muffler off, due to a failing Ignition Control Module (ICM). It may have allowed gas fumes to build-up in the exhaust system and then ignited them. You may have spark but it's not sparking when the PCM (computer) needs it to. Replace the ICM and the problem may be solved. Easy job.