In addition to what Bogie said, you don't have to worry too much about numbers matching on a '80 Vette. Dime a dozen, poor performers, not a lot of demand. You probably got a great deal on it because of those very factors.
Doesn't matter how much it was decked. I would pull a head and see how far down the hole the pistons are, that's the important dimension. Mount your aftermarket heads but do the math and figure out what gasket thickness to give you a good quench. Follow the head manufacturer's guidelines for how long a push rod to use with their heads and you're golden.