You need to use the original power assist steering gear box as it has the proportioning valve in it to operate the slave hydraulic ram that your car uses. From 1965 and up power steering took place inside the gear box and doesn't use a hydraulic ram. If you retain the factory original you can have it all rebuilt. If you can not live with the older style system you can use a mid eighties and up (such as the box off the Impala SS that you got the motor out of) and change the steering linkage parts to fit the new box's Pitman arm.
There are three people currently converting their 1958-'64 Impala's over to a mid eighties box and I have seen two lists of the Moog front end parts required to swap over.