You'll want to look for something flat- Ford escape comes to mind. Then you slide in the special fit square nuts and screw down the rack.
Don't worry too much about the load ratings as to your RTT plans- The ratings are a dynamic number for driving/bouncing down the road, and and are more concerned with vehicle stability than structural failure of parts. That said, always learn how new roof loads affect handling CAREFULLY.
So its for Gen1 double cab- that means the roof of the cap is about 5 ft long. Since the bars won't be all the way at the edges, max bar spread will be 48-52in. Do you really foresee needing to move them around? If the topper has windows (as most do), adding a cross bar in the middle wont help load carrying much- you're just passing that stress to the front and back anyhow.
I used a single pair of Yakima control towers on my Tundra and direct bolted the landing pads through the shell. They come off in seconds and go back up just as fast- super sturdy, too.