Whenever you get to the ranger station you'll need do a thorough gear check there. The ranger may not be very happy to do it at 7 pm. If you do come in in the evening, you'll probably want to camp at Miner's Cabin, the first campsite in the Canyon, however it may be taken by that time. Camping at Black Rocks will have you at the ranger station at about noon or early pm depending on how early you get out of camp, current, & wind.
Campsites are typically alloted on a first-come, first-serve basis, however you MAY be able to work with the rangers ahead of time if you expect to pass the ranger station in the late part of the day. They may bend the rules and hold Miner's for you rather than have all a situation where your party must run the rapids after dark just because there's no campsite available above the rapids. Call the river office (the same you called for the permit) and ask if there's a river ranger you can speak with.
The plan right now is to float loma to westwater ranger station in one day then westwater in 2 days camping at little d or somewhere
BTW - your permit will only allow you one night of camping in Westwater Canyon. There's a campsite on RL about a half mile above the takeout that's below the permit section, however its accessible by non-boaters so is typically pretty trashed.