I am switching a 327 for the original 6 cyl and putting in a powerglide in my 64 convertible. I also want to change from manual steering to power steering. What power steering units will work? Are any other years interchangable?

I have heard all 64-72 GM A-body PS boxes will interchange. There are other cars also that can be used as a donor. You probably want a faster ratio box to tighten up the feel of your car. I used a 87 Monte SS box but I had to use a different rag joint coupler and different lines and such. No big deal though.
Do a search and you should hit some stuff here.