Help identify please: power or manual gearbox?

But, why are people replacing the power steering gearboxes for manual gearboxes? Could you explain?

Joe, a lot of A-body owners replace their power steering gearboxes with manual gearboxes because they don't like the "over-boosted" feeling of the stock Mopar power steering box.
Yes, FirmFeel makes a great replacement product but you can modify a stock Saginaw PS pump to reduce the "over-boost" and give you the firmer feel. Check out this article from Mopar Action magazine for the "how to":
[ame]http://www.moparaction.com/Tech/beep/PUMP_IT_DOWN-re-v1.4.pdf[/ame]