$600 is too much for even a running 360. As mentioned above, if its already been rebuilt you know its not a standard bore, so if you're planning on rebuilding its just a core thats been bored already.
If you need it to drop in and go I'd still have to take a look at it, especially if it was supposed to have 50k on it before he got it. That tells me that the current owner didn't rebuild it, he was told it was (sketchy, this is now a 3rd hand rebuild). And it probably has way more than 50k on it now, which means its a rebuild candidate in short order. The 318 in my Challenger was supposedly rebuilt "2-3k miles ago" by the guy who had it prior to the guy I bought it from. It runs good and compression checks ok, but leaks oil from at least 3 different spots, and I can guarantee the accessories are NOT 2-3k old, they're original to either the '83 block that the 318 turned out to be or to the original 318 in the car. So, in short, maybe it was rebuilt, maybe it was a junkyard salvage repainted and called good, who knows. I'm running it until my 340 is finished, then maybe I'll take a look at it and go through it for another project...
There are plenty of 318/360's available for less, or in better condition. When I bought my 340 short block the guy offered me a 360 short block with extras for 50 bucks.