Dude, there has been a Zillion, yes 1 Zillion thread about this - use the search function.
For an R34 GTT use the bolt that holds the back seat to the floor pan on the passenger side foot well. Undo the bolt, make a hole in the bracket if there is not already one and bolt it on there, you can reach it from your drivers seat unless you're a mega short ass. This will work as I had mine mounted there for track days.
If the fire extinguisher you have does not have a bracket don't waste your time. Go to Kmart or Autobarn and pick up one that comes with a bracket, they are cheap.
Zing!