really depends on what is in it and the "history" of it. I have seen one sell (about two months ago) for 1550 and it was in good shape but had a 6cly in it and last summer my brother sold his "wreck" for 8500!!! but it had a Hemi (running bad) in it. SO what is in it? and is it original? any mods?
i sold my 70 340 duster about 2 years ago for close to 9grand, it was an original 340 car but had a race 360 in it roll bar, slick, spool, i could barely drive it on the street it was heavily modified in the drivetrain but the exterior was stock looking, i put on 1/4 panels and rockers and painted original fk5 adark burnt orange metalic it looked good and ran really hard but it was no show car..
if the car you are pricing is a totally stock 6 cyl car and there is minimul rust and all the trim parts are good you could get upward of 2 grand.. if it an original 340 car and its drivable i would bet you could get 4-5grand if not in to bad of shape. if its restored you can get likely over 10 grand depending on the restoration effort... look on ebay and get a better idea of whats out there.