I suggest you get yourself a gregorys manual for the car first off.
If you get lost with what I write below then I suggest you enlist the help of a mate who has done one before - almost every DIY mechanic from the 70s & 80s has done a holden 6 timing gear......
You need to remove the fan & shroud, accessory pulleys & brackets, radiator (to create enough space) and then the harmonic balancer needs removing with a puller.
One that's off you can undo the timing cover off the front of the motor, stuff a heap of rags down the opening to the sump, drill/grind/chisel the old fibre gear off (it has a steel centre which is best removed by drilling down each side & splitting it off the cam.
Then get a new alloy timing gear for the cam (about $40), line it up on the front of the motor so it and the crank gear have their 2 dots touching (you'll probably have to crank the motor over by hand for this).
Then heat the new gear up in the oven - yes the oven - for quite some time (like over an hour at about 170-180 degrees) while you clean all traces of crud off the front of the motor, and wash the front of the block & cam surface with thinners to remove all traces of oil (otherwise it smokes like buggery).
Then get the timing gear from the oven, using tongs & welding gloves, then line up the 2 timing dots, jam the gear on the front of the cam & firmly thump it home before it cools & grabs tight.
This way you don't have to remove the cam form the block & have it pressed off & on.
Don't hit it too hard as you can force the cam out the rear of the block (pushing out a welsh plug in the process).
Then assembly is the reverse of removal.