Gary, there are a few problems.
-The 78 won't have the three groove pulley for the AC.
-The 78 won't have the PS pump mounting holes.
-The 78 will have the low compression head w/ dome pistons, less power and efficiency than the 87 engine.
Buy the truck if it's cheap, and then rebuild the 87 engine or buy a used 81-87 engine and drop in.