They don't all sit on your factory roof rack; many safari baskets bolt directly to your rails on top that the factory racks bolt onto. I have a lightweight aluminum SURCO rack that holds a 100 lb 35" spare tire, full camping gear, four gallons of fuel, comealong, MaxAxe, HiLift jack, and whatever else I can throw up there for long trips. I also climb on the roof while all that other stuff is on there to strap it down, so add another 165 lbs on top of that. Con-Ferr steel safari racks are known for being extremely strong. There are many other good strong products on the market.
Any good safari rack that replaces the factory roof rack and bolts directly to the rails on the roof should be sufficient. Racks like the Yakima bolt onto the factory roof racks and break the factory racks in half. Sinjin (a prior moderator on this site) bought a tent from loftyshelters.com that mounts to the roof. It has a hard shell case in the collapsed state and then extends up just like a popup camper and holds two people to sleep ON THE ROOF (see below).
Stay away from the baskets that mount to the Ford racks and go with the ones that replace them, and you should be fine.