Your best bet would be to check what they are going for locally and on Ebay.This should give you an idea what the finished car would be worth.
Now for my opinion:
If you are thinking of building this car because you like it,then don't worry to much about value.The value will be the satisfaction that you get out of the car.Remember that this is a hobby of love.If you intend to sell the car when finished, then money is certainly a concern.
I look at it like I would if it were a new car.When buying a new car,I know that every year the value of the car is going to go down.I take that loss because I get use out of the new car.If your project car sells for less than your investment in say five years from now,then you could apply the same principle.
Good news is that every year that goes by,your Firebird should become more and more valuable.