Nice set up. How did you incorporate the torsion bar?
Subaru made it easy. It unbolts from the car with just 4 bolts, and it's all incorporated. This unit was from a 2WD Wagon with over 100,000 miles on it. I've used it in the woods, hauling logs for firewood. I had a Mutt, an M151 "jeep" - and it would drive right over fallen trees, and the trailer loaded with logs handled that just fine. It is truly excellent off-road, even with the teeny 13" rims/small tires of the original car. I eventually built a new topper and enlarged the deck to what you see now, and it handled running the Alaska Highway too.
Something to be said for OEM car-stuff - Subaru is a VERY common car up here in Alaska, and breaking rear suspensions is not a failure I have ever seen - i.e. lots of rusty, crappy, worn-out Subaru's but none dragging the rear-end close to the ground - believe, me, the owners wouldn't stop driving them for little things like no rear suspension

. For a 1000 pound payload, I think it's the simplest/strongest/best suspension for a home-built on a budget. The only downside is that 2WD isn't very common in Alaska, so spares would be tougher. A 4WD rear assembly would be nearly the same, and my friend has one we plan to make a trailer out of, one of these days

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As for coil-overs or springs instead of the torsions that are incorporated, I think if the load carrying capacity of the Subaru isn't enough, you need to buy AT's suspension.