In my opinion, I would just keep the original system. You can get a new Rebuilt or Replacement Steering Box for a pretty reasonable price. It would be a pretty easy install, everything will fit properly and it will save you a bunch of money. If you decide to go that route, you should be able to order one at your local parts store.
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America