Your 880 casting is vortec block. I have to assume someone has changed the heads at some point. That casting was only used for the vortec 350 with 062 or 991 heads. By the way i have 2 vortec tec blocks and both have the hole machined 4 a mecanical pump. They just had block off plates. aYou really need to follow Bruces addvice and pressure test the block..