I also posted this ad late last week on this site. I tried to contact him then, and as more often than not, no reply of any kind even today. Too often typical of Craigslist.
Probably why it says "mostly original" or he doesn't know the size of the engine for certain. Maybe the original 440 is in his Cuda now and he wanted to off the rest of the car since it is pretty nice and put the Cuda's old 383 in it so it would run.
The 383 is a red flag........ That and all the chevys parked around the 300. Not likely to get much accurate info about the car..... you'll need to go look.