Are 340’s rare

Can you clarify this?
Sure. If a 340 shortblock is $2000 (I'm assuming, I've never owned any small block except for the two barrel 273 from my 67 barracuda. I own more 90 year old straight eights than smallblocks!) I would MUCH rather have a $100 318 shortblock bore it .030, and put a scat stroker kit in it... and probably still have less in it than a 340.
And, anything you can put on top of a 340 short, you can put on a 318/392 and get same or better result.