Well, the 301 wasn't around yet in '78. Most likely it was a 305 Chevy. But being an Esprit adds a few other engines to the mix like the V6s. But the 305 was the more common one for them.
As Rick said, as long as the harness isn't butchered it wouldn't be too hard to just reconnect. You have connections for starter, alternator, oil and water senders, distributor and choke. Of course going from a Chevy to a Pontiac means some connections are in different locations or even on opposite sides like for the starter. This means shortening or lengthening some wires. An easier way would be to get a harness for a '78 with a 400. Then everything would just be plug and play.
