The big reason to go with the 3.4 is the owner gets to reuse all of his current 2.8L stuff (exhaust, motor mounts, engine accessories, etc). If most of this stuff is bad or the truck is rotted out, then selling the truck and buying a 4.0 is likely a better deal. But if the owner has a cherry 86 and the 2.8 finally gave up the ghost, then the 3.4 is a practically bolt-in swap and they get to keep their baby. :D