Yes, you can check in, get your resort ID's, parking pass, and head off to the parks.
And, if your site is ready, you can go ahead an occupy it, too.
Depending on how busy things are when you arrive, it is possible that your site will be ready.
I don't generally get to the Fort too early, but on one trip I did, and got my site before 8 am! On other trips, though, I have arrived at noon, and have had to wait until the site was released. You just never know. I think it's safe to say that you shouldn't expect to be able to occupy your site before 1 pm. If you arrive early, don't expect your site to be ready, but if it is, that will be a nice surprise.
TCD