Add up what you have into it including the car, subtract what you think the time/mileage you've used the car for is worth (I figure about $1,500 per year when I do this). Then think about how long you plan to keep it if you continue your build and how badly you want to keep the car. If the number you came up with earlier is close to (or less than) $4,500 and you plan to sell the car in the next few years then do it now.
If you spend $10K+ to finish your build you're going to take a bath if you decide 3 years from now you're tired of the car and just want to get rid of it.
So that saying "It's now or never" fits pretty well here.