hey mate
did you turn the motor over with the dizzy cap off to ensure the rotor is turning? if not, take the dizzy cap off and hit the starter quickly to see if it turns.
Also, are you sure you put the rocker covers back on the right sides? i can see in one of the photos that you have some sort of PCV port on the back of the passenger side rocker cover, it may be causing you issues if it's not breathing properly. I wouldn't think this is your only problem tho.
Next, when you turn the motor over does it turn over freely as per usual, or does it feel overly lumpy? (not talking about the firing side of things, just the mechanical fluency of the pistons moving up and down in the cylinder)
If it is lumpy or very hesitant, have a look at your oil to ensure it's not milky or way over full. if its normally at the full mark on the dipstick and is now well over the full mark - but not milky - dont turn the motor over. Drain the water out of the sump before you try anything else.
I doubt you have water in the sump, but better to get the point across.
What did you do specifically? you removed the rocker covers, but did you unplug anything along the way? did you accidentally pull off a vacuum line?
what motor is it? you say that you've matched the firing order to the holdenapedia site, but are you sure thats for your motor? did you only check the 2 wires that had come off, or did you unplug and replug all of the plug wires?
check all the earths in close proximity to where you've been working, make sure none are frayed or removed
anyway that should be a few more things to cross off the list, if none of this works, take some detailed photos of the motor so we can see what's going on