Crashtech, thank you for your honesty. I'm very much aware of what you refer to as the sunk-cost fallacy. I would call it "false economy". That is, putting more labor cost into a project than it would cost to buy a direct replacement and move on. Fortunately for me, this project falls more into the "hobby" category and allows me to use it as a learning tool. I am being paid for working on this car but I would never charge more time to my friend than it's worth.
I see it as a way to learn as I go and pick up new skills or improve old ones. For instance, one of the front fenders was severely damaged by some roofers that were working on my house. My friend found a replacement fender (which I reimbursed him for) but it's, in some ways) in worse condition than the original so I'm going to try to repair it, at no cost to him. If the result is good enough, he'll have his original fender and I'll have the skill and satisfaction. If not, the replacement fender will be used.
BTW. The first work I did to the Chevelle was repairing the windshield, front and back, trough. After that, replacing the floor pans then the trunk pans then both rear quarters. I'll have to say, though, that the hood is by far the most challenging.