2001 Chevy Silverado Multi Misfires (2001 Siverado Vortec)

Engine Mechanical problem
2001 Chevy Silverado V8

I am helping a friend to try and figure out why he is getting multi misfires (scanner does not point to any one cyl). He has replaced, MAS, O2 Sensors, Plugs and Wires. Everything looks good but seems like the timing is way off but there is no way to check it out since there are no timing marks.

The parts that he has changed were coming up on the scanner, now no other code is popping other than the general multi misfire issue.

This also happened suddenly as he was going down the road.

Any idea or troubleshooting chart to check?

Thanks
dave