Hi Cameron and welcome. A bit of a big ask there.
Why aluminum? Stiffness is about pound for pound with steel in load carrying situations. Steel costs less too.
Corrosion, well, aluminum has the advantage up to a point. Stainless steel bolts will cause galvanic corrosion and the trailer will fall apart. Steel can more easily be welded by the home builder and should never fall apart.
Axles have bearings and seals to keep grease in and water out so there needs to be some machining done. Best bet is to go to a website that sells trailer parts like axles and hubs. Far cheaper than getting your own machining done.
Finally to answer a little more specifically, for 400-pounds you could almost use well-aged Pasta as a building material. That is just 200-lbs a side so when you decide on a material, ask the questions again. The load you are looking to calculate is from the resting point of one hull at the end of the support and the point at which it crosses a longitudinal member, or looking at your drawing, about the 18" mark. That is the bending moment you need to define.
Then use one of the calculators on this website kindly provided by Kelly, and all will be revealed.