Power steering questions

Up until a later year model ( I forget ) the pitman shaft was the same small diameter so no you wont need to change the pitman arm.
The different pumps require different brackets and pulleys so I would definately find those first.
Several years ago I bought a complete pump assembly off ebay ( oval neck Saginaw ) that was junk yard yellow paint pen marked 68 a-body 383. I found it to be identical to my 67 273 assembly in every way, Go figure. We certainly can't go by what somebody wrote on it though. I wouldn't want to "steer" you wrong.