I highly recommend night fishing outside the park. I would advise anyone thinking of this to scout the area you want to fish thoroughly beforehand during the daylight hours. The river, as with most other places, can look completely different in the night. Nothing ruins a night fishing trip quicker than stepping off into a hole and possibly dropping your rod or worse! A little careful planning can produce a memorable after dark adventure.
As far as flies, I too prefer large woolies and streamers, something that will displace lots of water and make some noise. This is usualy the time when the larger browns come cruising looking for a meal. Also, be mindful of your backcast area, nothing worse than getting your favorite fly hung in a tree in the dead of night. This is another good reason to scout your fishing area and make mental notes during the daylight.