Transmit 7MHz analog signal from board to board
If you were to consider this properly, you would need to know the signal-to-noise ratio of the actual signal leaving the board and, how much that SNR could be degraded by the wires and input buffer before it becomes unacceptable. You would also need to determine what interfering signals may be present and how much the signal may be disrupted by a common ground between PCBs with the possibility of additive (but unrelated) ground currents passing down that wire.

That would be the theoretical approach and given that we don't know those details it's the only one that can be safely offered as advice. In the absense of that information I would consider a balanced-impedance transmission, screened twisted pair and a differential receiver. But you may get away with a much simpler set-up especially if the lowest frequency isn't 0 Hz as you state (you also state "passband" and that implies the possibility of an AC coupled signal.

You won't need to impedance match if the highest frequency content of the signal is 7.3 MHz however, if the signal might be a 7.3 MHz square or rectangular wave then there may be a need.