Go with Vintage Air. They have reproduced the old Mark IV, underdash A/C unit. They can set you up with a complete package of parts and hardware specifically for your truck. The underdash unit requires no dash modification and puts out lots of ice cold air. If you have a 4 spd. trans, you might have to heat and rebend your gearshift lever to clear the unit. I had to do that on my 69, W200.
Give them a call. I paid about $800.00 for my system 4 years ago. It is easy to install.
If your steering setup is like the one on my 58 Power Giant, which it probably is, you can add power steering with not too much work. I used the power steering pump, gearbox, and lower half of the steering column from a 1990 half ton, Dodge 2wd pickup. The greabox bolted right up to the frame in the same place as the manual gearbox, and a couple of the existing holes even lined up. The 90 steering column has a joint/coupling not far above the gearbox. We cut off the original column, attached the the coupling, (from upper section of the 90), to the original column, and fitted the original upper and new lower sections right back together. We also had to rebend the rod from the gearbox to the tie rod so the truck would turn fully to each side. That took a couple of trys to get it right. The new system worked perfectly.
hope this helps.
Jeff