[old Rusty C10]

the seat would interchange the tailgate would interchage using the hardware from the tailgate you want to swap the drivetrain would interchage (i think but being its 4X4 you should really wait for a response from one who is more versed with them as i am into 2WD) although you may have to do something with the motomounts and you may have to swap out the trans crossmember and the driveshafts/carrier bearing and of course you would have to do somethng about the transfer case so it may be easier to look for a 2wd crossmember and trans or just swap it over with the flex plate and go automatic
you need to usde the body mounts for the 84 cab and would have to switch the wiper arms and linkages from the 77 into the 84 cab.. you would also have to do the vin tag swap or show the state and they would have to issue you a VIN otherwise you would have issues come emmissions testing
there is alot of work here why not just swap your nose/wipers from the 77 to the 84 and makie it easy