I went through the same conversion about 30 years ago.
Yes you will need a different drag link (3 pieces) any GM 4X4 from 1972-1980.
The steering arm many are talking about will not bolt on your closed knuckle drum brake front end.
GM did make a power steering arm for a closed knuckle front end in 1967-68 but it is very difficult to find. (it is shorter than the non PS arm).
You do have the correct PS box and pitman arm.
You have a few choices.
(#1) find the rare closed knuckle steering arm and drag link (next to impossible).
(#2) find a front axle out of a 71-80 GM 4x4 with the same gear ratio as you currently have and put the entire thing in. (which upgrades you to disc brakes).
(#3) This is what I did.. I used two parts of a stock 71-80 drag link (which is just two tie rod ends and an adjuster sleeve). and dug into tie rod spec sheets and found a tie rod end that had the correct taper for the steering arm, the correct length, and the correct thread size/pitch to go into my adjuster sleeve. there is one out there, (I do remember it was for a 60's vintage Dodge) unfortunately I do not remember the exact application.
Here is a link that may help you find one.
http://www.moog-suspension-parts.com...e_Rod_Ends.asp