I concur with sleeping at 8,000 to acclimate, or consider Horseshoe Meadows/Cottonwood Pass Trailhead if you can sleep well at 10,000. If you can back in to a good spot at the Portal, it's not bad. Just show up, keep your options open, ask around, and things will work out fine - probably better than you hoped for. Just beware and be aware of the bears waiting to swoop in at any opportunity at the Portal. The campground host will know the usual suspects and their usual routine, but they can show up at any moment.
Have a great trip.
