Looks positively huge. <img src="/images/graemlins/smile.gif" alt="" />
Rattlesnakes are the only indiginous threat that I read about on the park's website (unless the sheep get agitated, you should be able to avoid them).
Water, and extra water and then some more water. I'd probably include a 2-way radio or better yet, HAM (I'm not an operator as yet). Decent First aid kit, with snake bite would be good. Make sure you include some sort of tent or at least a survival shelter with paracord to get out of the sun if something happens. A sealed bottom tent with zipper to keep out the critters would be ideal.
When in the sun, I like to wear a light color ballcap with a white cloth tucked in to catch sweat and to hang down over my exposed neck and ears. Chapstick and sunglasses would nicely round out your kit (I'd bring 2 pair as I've lost some before and it sucks). An extra pair or two of wicking sox and some moleskin (hot sweaty torn up feet are no fun). I would also include a clip on headlamp and backup LED light if you get caught out at night. I assume you'll have at least a folding knife if you read this site. <img src="/images/graemlins/grin.gif" alt="" />
Make sure you remain observant, make plenty of noise and let someone know your path, departure time and arrival time. This is also a good set of rules for snake territory (or really any territory I suppose):
http://www.wf.net/~snake/field.htm