340's are an interesting engine. They just have so much history and nostalgia around them. And prices vary a lot depending on whether or not the person that has it knows anything about them.
I bought my '68 340 short block for under $300, steel crank included. Now, it was already .030 over, but as 340's go it was a pretty good deal. I've also seen them go for twice that, even in need of a larger bore. And 5 minutes after I bought that engine, I probably wouldn't have sold it for twice that, simply because I found a 340!
Realistically, its easier and cheaper to rebuild a 360, and you'll have an extra 20 cubes. But, then you can't say you have a 340 now, can you?