I have had headers on 3 cars and have had nothing but trouble on all 3 cars. I think you would be better off with the stock exhaust manifold, duel exhaust, and low restrictive mufflers. You will only loose a small amount of top end performance, but you will have a lot less troubles.
If you still want the headers, you can contact Sanderson Headers. Contact them by mail or phone, they do not answer their E mail. They specialize in making headers for street rods and specialty vehicles and can probably tell you what would work on your vehicle.
If that doesn't work I would go to some car shows to see if anyone there has a truck similar to yours that you could look at.