for the most part it is a relatively straight swap, you will need to cut off the rear cab mount from the 86 frame and the 71 frame, install the 71 mount on the new frame. the frames are the same width in the critical places so all mounts will swap and be spaced correctly. the main issue will be the height of the mounts in relation to the cab and frame. while the frames are the same width they are not the same height and you will need to meak a spacer or custom mount for the front cab mount and the core support, this will be dirrectly dictated by how you possition the rear cab mount and what spacer you use there. its done all the time. it is far easier to swap a front axle into a 2wd frame though.
