Also check the frame behind the steering box, it needs to have the raised pads to bolt the box to, if it's flat then you'll need a frame spacer kit which are usually listed for 60-66 trucks. Like this...
http://www.classicperform.com/power6066.htm
You can find the adapter plate on the forum or other stores. I did this to my 68 K-10... also swapped the axle and put an add-a-leaf on the back for a 1.5" lift. Might look into power brakes since the disc setup will require new prop valve and lines anyways.