Re: Trailer Design Guidelines
Well, one thing is in your favor. The data from the coupler states that it is sized for a 2 5/16 ball. That's the size you'll need to pull this kind of load. And four wheel brakes would be required in most states. As the other poster stated, you can call Dico/Titan, but I'm pretty sure they will stand firm that if you're over 7500 lb GVWR, you need the 10,000 lb actuator. Since you haven't ever used your unit, you might ask if they'd give you a break on the price of a 10k unit if you return the smaller one.
3500 lb axles are not expensive, I've seen them under $150 with the brake flange and both hubs. But you'll be looking at more for 5200 or 6000 lb axles. I strongly recommend disc brakes- they're about $300/axle with cad plating. Make sure whatever you order has the same bolt pattern as your wheels. You have 6 lug wheels from the pictures, you'll need to determine the bolt circle diameter.
One last thought. If you're going to all the work to replace both axles, you should consider mounting both of them to an angle frame that can slide along the trailer side rails to adjust the tongue weight, then be bolted to the side rails.
