PJC built a great trailer.
Bkmaine2,
I like the looks of your trailer. It is a little heavy, but plenty long and can carry what ever you put on it. It would be good for hauling 6 canoes and some storage boxes. It can become your canoe club trailer.
My little 2 canoe trailer made commercially by Magento, has some small leaf springs. It wants to hop in the dirt and jump off the road. A bigger heavier trailer like yours is not going to do that.
Take your trailer on some dirt roads and watch how it behaves. I am guessing it is fine and better than my trailer designed for canoes. Leave the suspension alone. It would best to weld up your frames for boats. You could do it with lumber and carriage bolts.