I can offer dimensions of a similarly sized trailer that trails well.
Globe Trotter is 20' 11” long. Dry, weighs <3000 pounds, 400# Tongue Weight. The A-Frame is 36” From body to the ball and 32” wide at body-face of trailer cabin.
With five years of use, I've been very pleased with the Reese mini 350 WD System. It's a sensible, harmonious contraption. Mine is adjusted to tip 280# onto the front TV wheels, and 140# to the Trailer wheels. That's about 5% beyond its spec. It is quick and clean to set-up. Weighs 35 pounds including Bars, Shank & Head. The brake-shoe composition friction pads atop the A-Frame provide good anti-sway, and operate silently.
With regards to Lightweight trailers, what is pictured is very satisfactory in terms of easy hitching, anti-sway, weight distribution, minimal added tongue weight, quiet operation, excellent ground clearance, backs up and turns sharply without consequence. It has a relatively clean appearance.
I switched to a single 40# propane bottle. It's base is mounted flush to the depth of the A-Frame, rather than on top. It is situated closer to the body to reduce TW. It does not interfere with WD, and reduces tongue clutter.
I don't know what level of historical protocol you require, or If you're thinking of updating to a newer Marvel coupler, or rebuilding what you have, or a total changing of the A-Frame to trailer frame, or extending the coupler 6”?? If your coupler is good or rebuildable, maybe check out a few hitch systems for something compatible and preserve the original if that matters.
Anyway, those are the 1968
A-Frame dimensions for reference...
I'm guessing that you'll be towing at times, with a period correct TV.