Not sure how "prototype" you intend to go with the suspension, but set ups similar to this are easily availible at any trailer supply shop, here - ready to weld on.
I would turn the shocks 90', tho, across the chassis.
And definatly add a sway bar.
Cheers
Matt
This photo is excatly what I am going do to with my M416 trailer. How did it performed so far. What's the diameter of the sway bar would you look at?
Regards,