You own a metal shop! You'll have so much fun. I'm in the middle of making a bunch of stuff right now and it's super. These little things are so versatile. Make sure you examine a LOT of pictures, or actual implements, to see how they're designed, and where/how the stress is transferred to the the tractor. I take those as my model, because I know the old implements were designed to be safe for the machine.
I agree with others, you should just buy a drawbar. Some implements require that precise design, and almost more importantly, you'll need the drawbar brackets as your model for anything you bolt to those rear implement bolt holes. They have a particular taper, and I think you'd want to copy the original bracket for getting the spacing and taper correct.
The Cub rear lift system is the rear rockshaft ("normal") that attaches to the hydraulic cylinder by a bar, and lifts things up and down. The fast hitch is a special attachment, which gives easier mounting, but also takes up some space. As a farmer, I want to be able to drive over plants, and the fast hitch doesn't work for that.
The rear lift MAX is 600lbs, hung directly below the rear rockshaft. Don't push it. Be SURE you do the math on anything you create; a long implement adds significant weight/leverage to the lifting mechanism\, of course. Also be careful of weight and shock loading on the castings that you're bolting into. All the original implements, as I understand it, were designed so that it would break before the tractor would, or the tractor would stall before it could damage itself. It'd be pretty easy to make something that would be dangerous to the tractor, and to yourself.
Many people have made up their own 3-pt hitches, and in fact there was a thread on that a few weeks ago or so, http://www.farmallcub.com/phpBB2/viewtopic.php?f=1&t=75835&hilit=3+point+hitch