I found another site that sells them as well. The YT site won't let me post the link because they consider them a competitor, but if you remove all of the spaces in the line below and paste it into your browser it should work. The first one on that page says it is for all of the 4 cylinder models made from 1953-1964 including the NAA:
http://www.s s b tractor. com/Ford-New-Holland-tractor-parts/power-steering-conversion-kit.html[/url]
As WayneIA said, Roland Jackson makes a kit too. He doesn't list a price on his web site, but here's a link:
http://jacksonpowersteering.com/category/products/complete-steering-kits/