For an electric roll-down window...Hit the junkyards and find a totaled pick-up that already has one. I personally think it would be much easier to graft-on the rear cab wall from another truck, than to try and design a sliding window from scratch. It doesn't really matter what truck...Ford, GM, Toyota, etc. As long as it has a similar size and contour. It actually sounds like a pretty simple mod, if you can find the right donor truck. You will need to remove the H3T bed from the frame...but after that you should have full access to the rear wall of the cab. Could be as simple as a cut, trim & weld the new wall (with the electric slider) into place. Then paint and put the bed back on.
For the manual sideways slider...you can probably go to a custom truck accessories shop and find one. Or at least one that is slightly larger so you can cut it down to size.