The block I have is a 96-00 454 bbc, the casting numbers are 12561359, yes I am positive those are the numbers, this is just a block on the bench they were easy to read. I was told it was a gmpp block however neither the gmpp website or a broad google search yealded any results for those numbers, please help im ready to take it to the machine shop but I would like to know what options I have with this engin.