The only reason to use an 8wt for LM bass is to accommodate for casting larger, heavier flies. I use a 7wt and have never had a problem even when using 8 inch long streamers. That said, I don't use long zonker strip flies because it's as much fun as casting a wet sock. As far as backing, 25 yards should be sufficient. I have never had a LM bass get even close to me seeing the backing and I've landed some decent sized bass here in Georgia. One other consideration on your setup, if you ever plan to use it in some of the larger reservoirs where there are stripers, I would go for the 8 wt.

Jim Smith