I really would have a hard time putting a value on it, but can offer what my problems were. If the motor is going to be pushed hard, then there may be block flex at the top of the cylinder banks, making it tough to keep intake gaskets in place. The later castings had ribs cast into the valley that tied the cylinder banks together. So depending on whether you have the ribs up top or not will influence what its value is.
I had some minor cap walk issues, so I went with a center counter weighted crank, which stopped that from happening, and may have helped a great deal with the intake gaskets. I have continued to use just silicone RTV for sealant so I don't know if there would still be a problem.
At the minimum, you could have it inspected and sonic checked, to see if it will go to 4.500 or beyond. That will probably be the big question potential buyers will ask. The less you spend on it after the sonic check the better though, if you plan on selling it.
