Rally Dash

The only lights in an inst' cluster that will work without chassis ground are brake and oil warning if so equiped. Those lights get their ground from remote switches. All others including hi beam get there ground through the cast inst' housing and vehicle body.
So if your inst' housing does have a ground to the dash via the mounting screws, that is only one link in a daisy chain ground path. Next probable culprit is a ground wire that jumps from body to engine block. It should be near fan motor or behind right cyl. head. It grounds the body to the engine so current path goes across the engine to neg' battery cable. That jumper wire is often left disconnected at engine change.