If you use a 7075 block, it will certainly be strong enough. Given aluminum's thermal changes, I think the heat generated by the steering system would be enough to cause seal leakage and cause certain precision tolerances to be hard to maintain. Yes, steel sleeves in key areas could accomplish some of the precision fit up, much like steel sleeves in an aluminum engine block. Without them, you'll end up with a steering box reminiscent of a Vega engine.
When its all said, you'll save some weight for sure. Why didn't the factory do it, manufacturing cost and warranty cost. 7075 was space age stuff back in the 60s. It cost a fortune. Additionally, machining costs way more than casting and when you're making 5 million steering boxes, saving a single $ adds up fast. Plus, the warranty issues later could also cost big bucks should things not stand up to time and abuse. The Bendix FI systems of the '50s seemed like a good idea at the time, but customer perception, service, and drivability became issues that would take nearly 40 years to overcome.
