Check compression, if it's ok you may have a water port to valley leak.

If a head gasket is blown, compression will be down on that one cylinder and perhaps the adjacent one.

Sometimes you'll see coolant dripping out of a spark plug hole or spraying out when you're spinning the engine over.

I don't know what kind of intake gasket you're running, but shim stock steel gaskets don't always seal well at the waterports - even if you use the supplied by the gasket maker adhesives.
Most times a thin bead of Copper RTV around the water ports will do it, but not always.

I've had very good luck making intake gaskets from .030 Vellumoid paper or the US made brand from CarQuest .030 paper that says, "rubber & fiber" on the label.

If your engine has the stamped steel bathtub that's part of the steel intake gasket you'll want to trim it down so it can be re-installed as a bathtub only.
True with the late big block Buicks (400-430-455), not sure about the BBC's.