Are 340’s rare

rare, not really... can always find them for sale.. valuable.. for sure and will only go up in value. 340s have a mystical reputation which will never go away and guys will pay stupid money for one instead of a 360 for 1/8th the price. If you don't need to sell it, oil it up good and put it under a bench for 10 years and it will double in value... no joke. i don't get it myself though.
I think it is easier to find one now then 50 years ago. I bought a 72' Dodge Van, new and around 74', I began looking for one for it and it took until 79' to find one, but I had to buy the whole car (69' Dart swinger). The cars are now disappearing, but the motors are kept. I have had several since and I'm keeping one now, to install in a pick-up truck, Prowler or anything else unusual. If you never had one, you are missing out on a lot of fun.