95 Tbird LX V8- coolant leak issue

I have a 1995 LX 4.6 V8, 105k miles, auto tranny. I just replaced the water pump last week after I noticed the bearings in the impeller were whining. I knocked out the old one by tapping it gently with a sledge hammer, replaced with a new one (with new O-ring), tightened the bolts and ran the car to operating temperature. I filled the coolant that was lost from the work and saw no leaks and patted myself on the back for a job well done.

A week later, I started the car cold and the "low coolant" light came on. I immediately checked the reservoir and it was about 3/4 of a gallon low. I also noticed a profound leak coming from the area of the water pump (car was still on). I filled the reservoir and drove home (about 30 miles), keeping my eye constantly on the temp gauge and my fingers crossed. When I got home, I checked under the hood to find no leak and that the reservoir was still full. Using the car again an hour later, I noticed no difference. When I let the car sit over the weekend at home, I checked the reservoir again Monday morning, empty- I filled it again and left for work, still keeping my eye on the gauge.

It appears that when the engine heats up and expands (as any metal will do), the leak is contained. When it cools off, the leak comes back. I have checked with a couple of local guys and I have been told that the pump is bad, the o-ring slipped and/or there is a crack in the block. None have looked at this and have given me this advise based on my description.

Please note that the car was working well for a whole week between replacing the water pump and this leaking issue coming up. Also, I did not use any other sealant other than the supplied O-ring that came with the new pump. Is there a heavier O-ring to replace this issued one, or would I need to add a liquid gasket to the lip of the pump (between the block and the head of the pump)? What of the talk of a bad pump or cracked block?