I have ran into this on my car and several other 302's. I don't know if it is for a differnt configuration water pump or what, but our timing covers have like an extra little bolt hole right in the corner where you are talking about. No bolt passes through our water pumps into it. But mine and a few others I have seen sprung a little leak int he back of the timing cover there where the metal is kinda thin. I took my timing cover off before I found this little pin hole. I had the same symptoms coolant would just pool up in that little valley behind the water pump ontop of the timing cover. Ended up fixing mine with a dab of liquid steel. The other two I have seen do this you could put your finger down there and feel the little hole. we just cleaned them up really good and and dabbed some liquid steel over them let it set up and presto no more leak. Not saying this is definently your problem , but it is something good to check.