[GMH454]

That would be very unusual. I am no expert in these but they all have their own identity. Chevrolet had various casting numbers and stampimgs on these blocks. I am not sure if this applied to all of them but the one I have inspected closely had a casting number. If you can give the block a really close visual inspection and let us know what you find. All Zl-1s have a base value but this can vary depending on history of the engine as much as condition. Where did it come from?