Dumb question power steering pump shaft length

[truckin 79]

I need to change my single groove power steering pump to a double groove pulley for upcoming AC install. Can I use existing power steering pump that is in the truck and just add the 2 groove pulley or do I have to change the entire steering pump? I dont think it matters but I have a 74 power steering power pump(the type with nut on the end) pump on my 79, is the shaft the same length on 1 or 2 groove psp?