What are the wheels off of? They look nice.
You are doing a great job there; it all looks very nice.
One thing I was wondering about is just having a single plane of flat bar at the front tying the tongue members together. My habit would be to try and tie both the top and bottom of the tubes together; less chance of flex trying to crack a weld. If you are planning to have a tongue jack, maybe you could mount another plate across the top of the tubes just behind the coupler; perhaps similar to the way I did mine.
The fixed jack (as apposed to a swivel style) is a compromise: it was easier to mount in a central location (so no frame twist when in use), but to remove and stow for travel I will have to pull the foot off and unbolt it at the mounting flange (the mounting plate is tapped to make this easier using a single wrench).