I just had a titanium rear rack custom built by Black Sheep Bicycles in Ft. Collins, Colorado. I paid $350 for the custom titanium rack for my titanium GT Xizang bike. But the more I study the details on the thing, the more I realize I got an amazing bargain! Not a single blemish in the dimensions, machine work or welding. Perfect. And they tell me I can put a 200 lb person on it. Well, just yesterday, I put a 308-lb. friend on it and it was solid as a rock! The front wheel of the bike raised up, but the rear rack was solid. Moral of the story: Get a custom frame shop like Black Sheep Bicycles make you a professional, heavy-duty rear rack - just to be safe.
And do what you can to keep that front wheel on the ground!