Like Dale, I don`t like the look of the spread axles either.My 24 foot Wells Cargo has the conventinal axle spacing and has never experianced any sway or tracking issues. I am sure that towing with a F350 Dually and an equalizer hitch contributes to the good towing manners.
Since I bought my trailer used, the D rings were already in position, but it did come with 8 D rings already in the floor, which allowed me to determine which ones fit the best. I find the others handy for securing my pit scooter, generator and fuel jugs. One consideration, but likely not an issue with your Stocker (or 69 Mustang), is how close to the side walls the D rings are. With my old 85 Mustang Stocker, I could use a pair of D rings about 3 feet in front of the rear ramp door, but with the Fairmont, which is back halved and has 13" slicks, I had to use a pair of D rings further back, as the tie down straps would be pulling across the inner sidewalls of the tires. Using D rings closer to the rear door (or closer together) allowed for a straighter pull of the straps.
My trailer does have a roof vent, and it also has a plastic cover that is open and screened on the back side, so I can leave the roof vent open all the time, including towing, with no worry that the wind at freeway speeds will try to tear it off. Jamies new 20 foot Pace that Dale refered to, does not have a roof vent, but it does have a pair of vents mounted to the sides, one on the front, the other at the rear on the opposite side. Maybe something to consider if height issues prevent using a roof vent.
My trailer came with a left side "escape" door, which at first I wasn`t thrilled about, but on a hot day, with all 3 doors open, the airflow going thru the trailer really helps keep the inside cooler. Again, maybe not an issue in your case, but with my Fairmont, I had to make a plywood riser so the the front tires could be elevated high enough so I can open the drivers door into the escape doors opening. Otherwise the bottom of the door would hit the side wall between the top of the trailer fender and the escape doors opening. And speaking of temps inside the trailer, consider color of the trailers outer paneling. I looked at several new trailers before buying my trailer, and on a warm day, the inside temps between a black trailer, and a light color was considerable. I walked from a white trailer, into an otherwise identical black trailer, and could not believe how much hotter the black trailer was inside. On a hot day, take a look in the pits, and you will notice the white trailers generally have smooth straight side panels, while a black trailer will often have major warping of the panels. I am sure that once the sun goes down, the black panels will go flat again, but I have to think all that movement may not be great for how the panels seal to the inner framework or wood.