There was a guy near me who had done something similar, but he left the barn doors on and then built a ramp gate off of the back. I have not seen him / the trailer in the last couple of years or I would go get you some pics.
It looked good. I think he welded a piece of two inch channel onto the steel base and then hinged the gate off of the channel.
Are you going to make it a solid door like what would come on the ones that you buy with a ramp or is this one going to be out of expanded metal?