For small block and B/RB engines, use the OE pump. Use the Saginaw style on early cars and Federal or TRW style on later cars. CVF brand power steering pumps work well with these racks.
For Gen 3 Hemi swaps, the stock Gen III pump puts out too much pressure for the QA1 steering racks. Several companies modify new and used pumps at a reduced pressure (in the range of 1,100 -1,300 psi), including Bouchillon Performance and Holley.