Mysterious 1964 dart rag top????
It is possible that is was originally a 273 (most likely a 2bbl) with a 3 or 4spd trans. I have heard of early A-Body convs. with manual trans but I would have to guess that they may be somewhat rare but if the car is that bad the rarity might be a mute point. If the engine was in fact a 340 it obviously was not original to the car and was swapped into that car sometime in its life. If the body is badly rusted especially the interior and trunk floors, frame rails, inner fenders, inner and outer rockers and the quarters then $1000.00 is way to much for the car. You will have way more than that is just buying and replacing the sheet metal.
It sounds like nothing more than a parts car now and if it does have a 340 that most likely is the most valuable part on the car. I cut up a 273 push button auto 64 Dart conv. that was so badly rusted that the only thing keeping the car from breaking in have was the center tunnel and what was left of the outer rockers.
Good luck though,
Joe