Ok, here's my thoughts... if the motor is out of the car already, that's worth something (no time spent extracting the damn thing, and you don't have the get the rest of the car towed off when it your done with it). But if it needs a rebuild, then it should be considered a core, which brings the price back down. $600 is still overpriced for a rebuildable core.
The way to think about it is to consider what you would actally be getting out of buying the motor. Do you plan on using any of the extra stuff like the intake, carb, distributor, valve covers, exhaust manifolds etc? Will you be using the original crank & rods? Do you plan on using the heads? If you said no to any of these, then subtract their value from the cost of the motor and you'll know what the motor is worth to you. Still worth $600? And when you compare the cost of disassembly, hot tank, machine work, etc., and cost of new components like a new block, rods, bearing caps, etc., (especially if you're going with new heads), and it may make more sense to just source stuff new. If you're starting from scratch and want a full on motor, look at some the crate motors, long blocks, or short blocks out there, like from FRP, PAW, Speedomotive, etc. This can actually save you money overall...
As for places to search for a rebuildable 351W, keep and eye on the paper, local free sales paper, and pick up a regional sales rag... I see viable motors in those on a regular basis, often pretty darn cheap.
Good luck.