The 3,500-pound hitch rating probably comes with fairly low (10% common) tongue weight limit, so you may not be able to carry as much as you want if limited to +/- 350 pounds, minus the carrierís weight.
Older 19G came with 5.4L V8 with GCWR of only 13,000 pounds, which probably limited towing more than anything else. The chassisí load carrying ability itself is likely limited by rear axle load on single rear tires. Itís basically the same E-350 chassis as my van, except I have V10, which provides 18,500-pound GCWR. I can tow close to 10,000 pounds with tongue weight of up to 1,000 pounds, but thatís limited by what the resulting load ends up on rear axle.
Obviously, I can use a weight distribution hitch to carry higher tongue weight while reducing load on rear axle, but weight distribution is not possible if hauling a load on a cargo tray.
One of the main issues with 19G remains the limited 6.5 cubic feet of outside storage, which makes hitch cargo carriers popular.