if 86 floor pan is low pan and NOT a bolt in style you will need a 2" body lift or cut the floor up and weld in a 4wd pan area or bolt in pan area from the old cab.
use the body mounts from the year of the frame NOT the cab .
and 81-up cab = new front fenders and hood stuff. unless you do a few mods to fit the older hood/fenders/cowl panel on the newer cab. if i recall its faster to convert new to old rather than old to new.
otherwise farly simple swap over.
i have 2 k30 trucks 77 & 79 . both have 81-up style bodys swaped over. and fit perfectly. but i did a 2" body lift on 1 and 3" body lift on other to clear the floor pans for the 4wd tranny/tcase setup i was running.
try searching a little in the garage and use advanced search and type in my screen name and some info should pop up. been cover a few times on this site. but i think i got you the basics.