8" wrench isn't nearly long enough. 12" would be the minimum. Really a bench vise is your friend here. That's how we did it when I worked in a LBS. You'll laugh at how easy it is compared to the 8" wrench.
I remember trying to get a freewheel off with that size wrench when I was about 16. It took about 30 minutes of grunting, sweating and two pairs of gloves (used together because the wrench was killing my hands) before I got the freewheel off.