I use a 9' 5wt on a regular basis in the Park. Abrams is a pretty good size piece of water (by GSMP standards) and the 8'6" will be fine.
I do have a 7'6" 3wt that I use mainly because I like the rod and it's fun to fish that little short light rod on the streams.
However, I wouldn't go out and buy a new rod just for the park if I already had a rod like you described.
Of course, while you're in Townsend, stop by LRO, talk with anyone in the shop, take a couple rods out back and cast. I'm sure you'll fine one that you NEED and cannot live without.
Should your wife ask, I will deny saying that and ask for written proof.