For a vehicle to be worth top dollar it must be in top condition with no aftermarket accessories, and the original paint color.
Contrary to what has already been posted there is no way your vehicle is worth the figure they posted. It is nice to dream of getting $25000 for something you bought for $3000, but it just ain't gonna happen.
Therein lies the problem. You paid too much when you purchased the vehicle. If the car was"wasted" away and needing a lot of work to bring it back to decent driveable condition then it was worth less than $1,000. If it still has original interior then the parts that came with it were not that much expense for the previous owner , so allow another grand. Making the maximum it was worth when you bought it $2000, and it should have sold for less than that.
What you have now is not a museum piece, but rather a nice daily driver.
It is nice to see classics being kept alive so that people can see wht cars used to be like. You bought it, you put your time and money into it, you painted it to suit your taste, you put on accessories you liked. Keep it, drive it and enjoy the daylights out of it.
Value now?? If you must sell it, start by asking $10,000, be prepared to come down. Be happy if you get somewhere between $5,000 to $8000.
I see Mustangs for sale all the time on the internet with asking price of that range.
If you are looking for a value to insure it, find a qualified appraiser to put a value on it for you. Then insure for 125% of that apppraised value.
Bear in mind a value for insurance is different than a value to sell it for.
Again- enjoy your classic and have fun.