I put a F4i rear shock on mine. I modified the mounts and also had to to some grinding on the frame around the lower mount.
The F4i shock has the correct spring rate and is identical in height as the pathetic shock that came the bike.
I also resprung the front forks and installed the racetech cartridge emulators. I also put on a fork brace I bought from China for $10 that was a perfect fit.
Do both of these and your FZ6 with amaze you.