So what's this '71 Demon worth?

Give a look at what real 340 Demons are going for in similar condition. Real, numbers cars. I'm "GUESSING" you could get "one of those" in the 20-25K range. What you're talking about here is a slant six car. Always has been. Always will be. THAT cannot be changed. Physically, yes, but the car was born a slant six car and that's what it is. PERIOD. So that must be taken into consideration thinking about price. "I" personally think 15K is overboard. By how much, I would need to inspect the car to know for sure and so should you.