Woody - Just a different opinion on my end...
Using the correct outer lift arm will not only give you more lift height but it will raise the blade faster due to the different length and angle. When I compare the Haban on a Super to the blade on a regular GT, the speed and height are something I consider to be a big advantage.
You already have the correct tie strap to add the extra support to the outer lift arm and I believe it is plenty strong with the tie strap in place. I see the outer lift arm being more susceptible to damage with the snowblowers due to their greater weight. I use the Haban tie strap with both attachments.
You are probably in greater danger by far of damaging something with your present setup, as you may run out of float with just the rockshaft. In addition, due to the different lift arm position, the angle with your current lift tube is also severe at the top of its travel. Lastly, I am surprised you are not hitting the tractor frame at the highest point.
If you had a duplicate of your tractor setup with the correct lift tube to compare it to in use I doubt you would go back to your present system.
If you decide you want to make one I can get you pictures and dimensions. If you have access to a shop press you would just need a length of the appropriate tubing and crush each end flat. Drill the right size hole at each end of the slots at either end of the tube and then use a Dremel with a cut-off wheel to connect the two holes to make the slot. You could probably get away with a large vice and the careful application of a hammer to flatten the ends if you had to. If needed, a propane torch will heat it enough to make it easy to flatten with a hammer. (Who doesn't like a little blacksmithy work from time to time?
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There are other ways and materials, but I think the factory tube would be the easiest to recreate.
Again, just trying to help, let me know if you want the pics and dimensions.