I'd go with gooseneck. It can handle the weight. Also Northern Tool offers blueprints for trailers. I'd pick up a set for something like a gooseneck flatbed. That will help get the angles right for the tongue, down tube and all that. Maybe the car hauler plans, to keep the profile low.
Are you planning to repurpose your dump insert? Or use the hoist parts to make the side dump?
Where the dump is going to be place, and with a side dump I’d cross brace the frame every 4 foot or so and then do a “x” brace to help with the twisting when the dump is being used.
If you want to use the old trailer, have the gooseneck frame run on the outside of the donor trailer. Then bolt the donor into place. Then use a 6 or 7 K torsion axle. Torsion axles will hold up and not need the maintenance a leaf spring system needs.
While I think your design is well thought out. With the side dump towards the front and tools and equipment to the back, I would be worried handling when pulling. With all the weight to the back, it might be squirrelly. You might be able to get around some of that by moving the axles back some. It would mainly depend on the size of the side dump.
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