Where can you find headers for a 69 dart with a 383 engine?

there should be quite a selection since the dart is the A body and the 340 is a LA motor or a smallblock mopar engine. the 273, 318 and 360 ci engines are of the same smallblock family as your 340. just a bit of advice , installing headers on a dart or other A body cars is a little time consumeing, the drivers side engine mount will have to be unbolted ,possibly the passenger side to,depending how much room you have on the drivers side,power brakes Power Steering,etc. then the drag link will have to be unbolted and possibly the transmission linkage if its a auto on the column. the drag link will go thru the headers between the pipes. the passenger side header will slip in place very easy,its just a little work, but boy they sound great when their on.