The only reason you will need 6-7x would be low and clear h2o, You shouldn't have that problem when you arrive, but check the board now and then b4 your trip for stream conditions. Leader thickness has been more crutial when throwing dries for me than when nymphing. I think people place too much emphasis on all this. Proper presentation is the key in the Park. I know people who fish with 4x and catch fish all the time.
As far as what bugs to throw, a trip into LRO is much advised. The staff are all very helpful and you will definitely catch more fish using what they have. Some of the locals tie their version of some popular patterns, those are what I try to buy. Once again, presentation is the key. Don't overfish a hole. There is a lot of water to cover here. Don't waste time going for the 'perfect' cast over an area where you've fished for the last 15 minutes. Move on and save that cast for fresh water.
Good luck and leave some for the rest of us.
PS An 8' rod is a good length. Not too tall, but still long enough for some highsticking.
May you fish more than you wish for.