440 or built 360 71 demon

Well, as expected, there has been good interest in this topic - about fifty/fifty SB vs BB. Ultimately it is up to the owner/driver as to what engine will sit under the hood. Today you also have to look at the fifth digit of the serial number. Now that we know it's a factory 340 car, I would ask owner how long he expects to keep the car. Reason being if he ever sells it, the big $ are commanded by 'numbers-matching' collector cars. Putting big money into something with incorrect engine (or body color) will not net the seller top dollars. Not only that, there is very little interest out there for vehicles built (modified) with one person's preferences. Another consideration is where and how a car will be driven. Also, car shows are a great way to have people drool over your ride - and offer tons of compliments. Most who look under your hood notice the visual quality and perfection --- and not whether or not said engine is a big or small block. Us 'Moparites' notice the physical differences immediately but, like I just stated most people who browse thru car shows don't know about OR care about engine sizes.
Again, good luck with what you decide and last advice is to keep the car drivable. - lest you regret it on the street.