You can take advantage. if a 10x1 shimano freehub type axle

the added spacer on a Shimano Tandem axle ( they aim for 145), and adding the left spacer moves the hub to the right, = less dish

less Dish makes a stronger wheel, when trued up again, the rim is more centered between the Hub flanges...
at as stated 400 pounds you can use a strong rear wheel ..

maybe bump up to a 40 or 48 spoke wheel.

My Touring Load rig used a 48 hole wheel ..