Water in oil after rebuild - looking for guidance

A few thoughts to add

Just to rule out any head or head gasket issues you might want to perform a compression test.

Do you have oil and water mixed in the radiator?

If it was a junkyard motor were the heads surfaced? Maybe the intake side was not milled as well and it’s not sitting properly. Not all shops follow this procedure.

Some intake manifolds are fussy with the type and thickness of the intake gasket.

Wonder if they pressure tested the heads for possible cracks?

If the intake is not sitting properly on the head you could be getting a leak at the bottom of the mating surfaces.

If you remove the intake and it pulls right off you know you didn’t have a good seal. If it comes to that look at the gaskets to see how they crushed.

Hope it’s something simple.