Alright...ignore my last post...did some more research...YES it will fit as long as you use the 76 mounts to mount the cab...
Hi Rusty ! I hope that's your name and not a description of your truck . Back when I were a Farm Laddie in New Hampshire (mid 1960's) I had a rusty F-100 from Ayers AFB , there was no floor left so I used old license plates to make a nice new ones... . Those were the dayze . BTW : I don't know this series of GM truck bodies especially well but : the hoods change mid way through , if the hood has the louvers stamped in it (your '76 should) I'd closely measure the '86 cab before undertaking this job unless you're planning an entire body swap , front fenders , core support and all . Keep asking away , these guys know everything about these trucks ,they're just shy .