You asked for opinions, here is mine:
Determine what a comparable car with clear title and current registration would sell for that is completely done. Then determine what it would cost to finish your car to that level. Subtract the cost, including labor, to finish your car from the value of the done car and that would be the maximum your car would be worth to me.
If you want to sell a partially completed project car, be prepared to for a reality check that includes not necessarily getting your "investment" back.
*Principal wrench on this 69 Mach I*