Since you know what the valley pan is, I'm going to say that you're talking about the IP housing/IP gear housing. You can't be leaking coolant from the IP gear housing. There isn't any coolant underneath/inside it. It is possible that you could be leaking coolant from the valley pan. IIRC, there is some coolant ports in the heads underneath it. They are sealed off by the valley pan itself. They were plugged off on the 7.3s. It also may be that the later 6.9s had those ports plugged off, but I don't really remember those details. You may have to install a new valley pan. If it's leaking externally, then it's also possible for the coolant to start leaking into your lifter valley which would then find it's way into your oil. This does need to be fixed.