Well if that is the case that is the limit on the frame of the trailer itself. I would guess you would need to beef it up some or start over and custom build a trailer using the 3500# axle you have.
Chris
I do plan on beefing it up quite a bit, along with adding a new coupler like mentioned. I dont really see the point of starting all over iwth a trailer, so im just going to use what i have and make it stronger.