Backcountry sites in GNP do present a challenge, perhaps the greatest you'll encounter unless you come face to face with a sow grizzly around a blind corner. If you can work it out with your dates of interest, I would highly recommend using the advanced reservation system and applying before the main lottery deadline (probably April 15 in 2011, as in past years). Allow your reservation to be as flexible as possible by selecting the following options on the application: different start/end dates, doing a trip in reverse, etc. Leave comments if applicable (e.g. if primary goal is to go through/to a specific site or trail, or any comments related to trip flexibility). The advanced reservation costs an addition $30 (if you get your reservation, otherwise there's no charge), but it's worth it, especially if you are traveling from afar.
However, the advanced reservation system is not the only way to work the system and many do not get a reservation. I haven't seen the numbers on success, but I wouldn't be surprised if it were around 25% based on anecdotal evidence from others I've met on the trail in GNP. You can get walk-in reservations, and many find success this way since 50% of the sites are set aside for walk-ins. You can make a walk-in reservation the day before you start on the trail. However, the rangers I spoke to this summer said there's typically a line at the backcountry reservation office (in Apgar anyway) long before it opens at 7:30am (I saw a line 3-4 parties deep at 6:30am on a Sunday in mid August this year). So get there early. The rangers are really good about figuring out the best trip with what sites are available, if your original plan is unavailable.
[side note: Have plans for a backup trip outside the park ready in case you're unable to find a suitable alternative as a walk-in. There are great backpacks in the Bob and Great Bear Wilderness just south of the park.]
My understanding is that some camps can't be reserved until 8/1 because the trails or camp sites are often not cleared from winter snow and debris before then. This coming year may be particularly bad with the La Nina predicted to drop extra moisture. So, if you're coming in July, expect snow in the high country. If sites are not open for advanced reservation, you may be able switch to them when you pick up your permit (if they're not booked), so consider selecting adjacent sites to those not yet open (so that the rest of your trip is reserved at least). An example of this would be reserving Kootenai Lake but switching to Stoney Indian when you arrive. If you can't get it, the trip will still work over Stoney Indian Pass, it just might entail a long day. If you get an advance reservation, you can switch things around just like a walk-in when you pick up your permit (at no additional cost).
You may have more success starting early in the week, say Monday or Tuesday. As for your specific question about the 70 mi loop along the Highline, it's certainly possible, but is quite popular. Advance reservations will be difficult to get.
IMO, this park is best explored in long trips, but they can be logistically challenging. This does keep some folks away, but the backcountry trails seem to be well traveled none-the-less.
Hope that helps.