Can small block oil pump be replaced without pulling the engine?

You can, sure. IMO though, and we all know what that's worth, it's much MUCH easier to get everything to seal going back together with the engine upside down on an engine stand. Laying on your back, upside down is just a pain trying to get the pan gasket and end seals all lined up and get them to defy gravity and stay put. But if that's how you roll, go for it. lol