I would recommend something more like this for the rockshaft arms, as what you've drawn will waste most of the lifting power with badly-angled linkage. This is better but still not really an ideal solution. This piece is going to have to be REALLY strong because of the L-shape.
hitch.jpg
The biggest problem here is that the seat bracket is made of steel that is only ~.100" thick, and the 3-point pivot pin is only 3/8" in diameter. The loads imposed on this rockshaft by a heavy Cat-0 implement will eventually crush the holes, and a 3/8" rockshaft will not be capable of doing anything. The OEM Cat-0 hitch has a 1" diameter rockshaft, and it pivots in two 3/4"-ish wide bushings welded into the hitch plate. You could bolt on a piece of heavy angle iron with a hole drilled to match the hole for the pivot pin on each side of the seat bracket to add some more bearing area for the rockshaft, but that won't solve the 3/8" rockshaft problem. You'll need at least a 3/4" diameter rockshaft to be safe.
I would also recommend tying the lower bracket into some of the axle tube bolts, as you don't really want to be dumping all of the load from the hitch into the trans by putting the rear cover bolts in tension. Yes, this is how the IH 3-point works, but notice that axle tube bolts are used in the thread you linked to, the picture CADplans posted, and also the OEM Cat-0 hitch. It's much better to load bolts in shear than in tension.
One last thing, what are you going to mount the hydraulic cylinder to? That mount needs to be very strong and mounted to something solid.
If I were you I would think this through a little more before you start cutting metal. I learned early on when I started making stuff that time is saved and less material is wasted when the item you are trying to make is completely thought-out before it is made.