re: rack on a tube axle
I just reread your posting. You might want to rethink filling the tube with grease. The grease in the tube would be non-compressible. Any movement would go through the whole assembly right up to the steering wheel. Even with out the grease, with nothing but air in the tube, there is still going to be a lot of movement and stress on your brackets. I hate to say this, but about the only thing I can think of would be to use a flexible shaft like those used on the Pinto's. With a torquey big block, that could be real trouble too.
I'm sorry, I don't mean to rain on your parade. I just want to present another point of view.