Are 340’s rare

You can make any engine perform well. It is all about how much money you want to spend. I know of a Senior in Texas that has a 225 slant six that runs in the 10s in the quarter mile and if you take a standard bore 4.04 340 and put a 3.58 360 SB stroke in the block= 367 C.I.D. and everything else is the same---the 367 will win the race.