I like Factory Power Steering, but I guess it depends on how you are building the car and what you will be using it for.
In '59, the PS set-up still had the Pump on the back of the Generator. It is an easy swap though if you stay with the original.
Take note though, if you change to an aftermarket, research it good first as many have experienced problems with them. The problems are also known to sometimes "snowball" with one fix leading to another one needing fixing.
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America