You should have a Prestolite dual point as the guys mentioned earlier. They were used in the 340`s up to 71 and the low compression 340`s used the electronic ignition/distributor. The manual trans. dual points had a little faster advance springs than the autos. The dual point gives a hotter spark at high speeds, you`ll need a dual point (at least) or an electronic ignition to give that 69-340 what it needs.
Usually one of the big meets like Carlisle or the Mopar Nationals will have the greatest concentration of vendors in one place. Your chances of finding the correct one would be the greatest at an event like that. There`s always Ebay too.