Hi,
Sounds like a nice car but it's REALLY hard to guess it's value. The best thing to do is throw away that price guide, those things are nearly NEVER accurate or current. My guess is between $5K and $20K, too many variables to get any closer, most I've seen sell for around $10K on average with descriptions similar to yours.
Bottom line is it's worth what someone is willing to pay.
Watch ebay for sales of similar cars (options, miles, condition,etc. being equal or similar) Convertible Challengers are popular,they are more common and not quite as popular as Cuda's but still in the same price ball park (I'd figure about 25% less than a Cuda with similar options)
Your car being a base model Challenger and not a "performance" model makes it a less popular choice and will keep the price down a bit, if it's an original black car that will help, if it's had a color change that will hurt it. Options like a Rallye dash, Pistol Grip shifted 4 speed, 6 way drivers seat, Performance hood, High performance engine (318's don't qualify) 15 x 7 Rallye wheels, and power accessories like windows, top. etc. will help it.
Also the word "restored" can mean VERY different things to nearly anyone you ask, restored is a very subjective term and is often overused to describe a car that has had nothing more than a basic "freshen up" job including a fresh set of seat covers, new (usually low quality) paint, and a quick "fix anything broken and not much else" engine re-build.
Most consider a truely restored car a car that has been completely re-built to "factory new" perfection. This involves a 100% "every nut & bolt" rebuild, all components would be either refurbished to "as new" or replaced with original or NOS parts from bumper to bumper, top to bottom. These cars will obviously bring ALLOT more money due to the investment involved. Just about any car restored 100% can cost 20K or more if done properly, and that would be a budget job with you doing most work yourself. Of course you need to consider the cars eventual value before such a restoration investment is made, a 318 Challenger convertible may not ever be worth the cost of a a full on restoration when it's finished. Most would buy a car like yours with intentions of "cloning it" into a high performance car so getting a good starting price for their (very expensive) project will be important to them.
another thing worth mentioning is a car with zero rust (and I mean ZERO in it's entire history) a ZERO rust car will maintain a higher value because examples with absalutely no rust EVER are very rare.
The majority of rust repairs reflect short cuts and eventually the rust comes back, hence the desire to have a car with no rust history....New paint with bubbles erupting here and there like around the wheel well openings are a sure sign of previous (usually shoddy) rust repair.
At any rate, hope this helps some
good luck!
