Finding the right combination of rack and mounting system is a complicated task. If you intend to use the factory roof rails, I suggest you question each manufacturer individually on the matter.
Front Runner, for example, supplies their own short rails, and up front a bracket/foot sits on the roof gasket over the point where the factory long rails bolt on. They have an installation video:
Front Runner Roof Rack Land Rover LR3 LR4 installation vide.mov - YouTube
I think Hannibal uses 6 or 8 feet and no rail.
It would be interesting to ask either Hannibal or Front Runner if they could supply mounts to accommodate the short or long factory roof rails.
Voyager Racks will custom build to your application if they don't have a ready solution. Just email them.
Others have had success using a hybrid approach. There's an example
here.
An interesting aside: the example above is on disco web.org, but for some reason whenever I type disco web (without the space) and then submit it, this site replaces disco web with "********" So I had to go to tinyurl.com and make a surrogate address that this site wouldn't censor.
I hope that helps.