IF I were looking for a '72+ Challenger and could get anything that was truly rust free, original & straight the value would be in my opinion much greater, say upward of 12K
However most everything I've ever seen was none of these things....Sooo even if it's above average & otherwise just as I perceive you describe I'm thinking you might expect to get maybe 4K.
Locally I was shown a picture of a roller with no interior or divetrain(and probably needing everything else) described as straight & solid, they were asking $2,500.
I would value a complete and driveable piece quite a bit higher as I could see what was missing and just how good or bad things are.
You did not mention if it's a Rallye(only that it has a Rallye dash) which might be worth something to someone wanting to do an OE restoration.