Did u end up trading for that 87?
To answer your question about the 79 panels fitting on 87 doors I am going to say most likely not and the panels will clash with the 87 dash pad.
With replacing the whole cab...IMO, the only time this is necessary or worth it is if the rockers, cab corners, floor pans, braces,.... Are completed rotted and you find a solid cab that is rust free.