my 390 had 94k on it when i pulled it, it ran with just a slight noise at higher rpm due to the bearings just plain worn out, also found 3 broken piston rings, and a very sloppy timing chain. so an all original 390 that you cant hear and have no history of..i say $600 is pretty steep. i got one out of a junkyard for $200 and had to bore it out, grind the crank, new valves, etc, etc.
any fe series engine will drop right in to your car, most common(cheapest) being 352 and 390.
68 Ford Country Sedan Wagon-390 FE, C6, 9 Inch
87 Chevy Caprice Classic Wagon-307, 2004R, 10 Bolt, what can i say, i grew up with that car