you can put a rear crawl through window in 69-71 trucks with a little work and no modifications to the truck. this window is from the next generation trucks.

you have to shave a little more then a half an inch of crown off of the top of the frame , it's just a plastic piece jamed in the frame . a hand planer does a good carefull job.

and on the bottom corners you need to jam some crecent moon shaped shims of 1/8" aluminum into the frame, just tap them in there with a hammer. there a groove all the way around the extruded window framing its easy to work with. a disc grinder a little here and there as you try to pop the window in for teh rite fit a few times. this whole thing takes a few hours. and it great in a truck with no AC