Are 340’s rare

Be careful what you wish for. The 408 may not be what you think you want. I have a friend with a 396 camaro and a 408 dart. Both engines are very similarly prepped and geared. Now he raced his cars numerous times against each other and said the Dart always beats the Camaro off the line but by 80 mph the 396 passes and walks away from the over square 408 Dart.The 396 with 3.75 stroke just flat out revs quicker. The 408 certainly makes more torque if that's what you want and may be better suited to a heavy vehicle. The Dart had 3.23 and the Camaro had 3.31 gears.