Need some advice

well....JunkYard cores cost $150-$300....for unwanted year 318's/360's.
If it is semi-fresh (less than 10,000 miles)...has valve covers, intake and carb, front timing cover,harmonic balancer, and water pump and a oil pan on it....I doubt you could get a better deal.

Machine work, and assembly are quite expensive.

Personally..... I would not pay $600 unless I heard it run and seen a compression check of all 8 cylinders.

Questions to ask:

How much Machine Work did the block get (heads been deck'ed?, cylinders bored, line hone, new cam bearings?)
What internal Mods does it have? (HV Oil Pump, Timing Chain tensioner)
How many miles are on it?...how much run time?
What else is included with said engine?....mounts? Flex Plate? Exhaust Manifolds/Headers? Extra's?