Take some advice from me, I have mounted so many cars on spinners I couldn't begin to list them, everything from a full frame caddy on a custom rig, to pickup truck bodies only, and I have 2 b-bodies upside down right now!!!
Do not over think this, just go to a steel house and buy medium wall square steel tubing.
What I would do personally is this,
go to home depot, grab 4 10 foot lengths of 1 1/2" black iron tubing, that has a od of 1.90", and 2 couplings.
Make them into 2 20's, lift the car up to put it on the rotor, fit the tubes in the holes and drill right through them, put some nuts and bolts through the square receiver and the pipe on all 4 connections.
That's it, it wont go anywhere, if you are worried about it sagging, then grab some exhaust clamps and clamp a couple casters to the middle of the pipe. But I don't see it happening.
That connector doesn't hold any weight it keeps the 2 ends from closing in or opening up depending on your lift points.
I guarantee the tubing will work.
B-bodies are easy to mount, 2 square plates in the back on the bumper mounts (right on the body), and two pieces of steel in the front that run along the sides where the front bumper bolts mount.
I seen a homemade twirler, with chains on the left and right keeping the rig from opening and a 1" pipe down the center to keep it from closing that was just stuck in there held by the chains keeping it tightly closed to the pipe... They had a chevelle on it, moving it over a crushed stone driveway, up a slight incline pulling it with a zero turn cub cadet lawn mower!!!!
So don't over think it, just go grab some pipe or square tubing (its hard to move 20 foot lengths of anything, I will guarantee if that's not home made the old rods were multiple telescopic pieces.
I wouldn't go redesigning the unit and I have both of mine with center legs, I want to say that was to cut costs, because the 2 legs would be a lot stronger IMO.
Good luck, take it or leave it that's my opinion and what I would do, I always take the advice of the guy who has been there.