Well a 318 is a very common motor. A 340 is a better motor and I'm sure others will argue with me. But an original 340 H code 73 car is worth more than the 318 car. Also the 73-76 Duster is all the same and not as desirable as the flat nose 70-72 cars.
If it's got some rust issues in the floor, needs interior work, no exhaust and who knows what else, start low, like 2500-3500 low and work from there. I don't think it's worth 5k but other might have a different opinion.
