Carter 4070 fuel pump

Here's the question..........Does it really make any difference if a Carter 4070 fuel pump is mounted vertical or horizontal? I have mine mounted vertical, and my friend just installed one in his BH, but mounted it horizontal. I told him I thought it should be mounted vertical, but I couldn't tell him if it really makes a difference. So I thought I'd put the question here, before he "buttons" everything up again. Has anyone got theirs mounted horizontal, and if so, have there been any problems? Thanks!!