When I ordered mine I called a torsion bar axle design engineer as my 76 Soverign had 3500 lb axles and GVW of 7100 lbs. He recommended axle loading to handle 50-66% of of the gross weight and recommended 5200 lb as the replacement which is what I ordered as that weight comes standard with 12" brakes. They can be ordered with increased down angle to raise your rig but normally they have 22 1/2° down angle.
Forgot to measure them with angle indicator yesterday when I put her back under her shed but I estimate they preloaded and rotated the axles about 15 to 17 degrees so there is some wiggle room for the big bumps.
The ride is very nice and while I wouldn't try it with anything expensive like a TV, computer etc I have left stuff on the front fold out tables and the galley counter top and after 455 mile (each way) trip, most of the stuff is still on the tables. I had a staple gun slide off running I-40 between Asheville and Knoxville which has lots of curves with recommended down speeds for 18 wheelers of 45 MPH on several of them.
It is 88 miles to the interstate system from where we live and two lane rural roads 80 miles of it and meeting 18 wheelers head on has no effect on the trailer I can feel in the pickemup.(2500HD)
I had to relieve the side plates as the square tubes are larger than the orginal but I used original holes. I had to put 1/4" plates on top so the load rests on the top of the axle plates but is secured from the side so there is no load on the mounting bolts.
This will give you an idea of how much the old axles had taken a set. Best I can figure it raised her about 1 1/4" with it on the ground.
Note the scribed line outside the original axle housing hole. This is how much I had to relieve the side plate. Note the factory bolt holes were not changed.
Check out my thread and new axles for pics.