lower end knock

Well yesterday was the day to finally turn the key on my 63 convertible. Rebuilt 200, rebuilt C4. Had everything ready to go. Car started right up, ran it for 20 minutes to break in the cam, oil preasure was good, water temp hung at 190. Shut it down, let in cool for a few hours while I checked for leaks etc. Started it back up ran great until it got to running temp of 190-200 then when I hit the throttle I get this knocking sound. (note I still have not driven the car yet) It doesn't to it at idle only when I rev the engine. The thing I'm confussed on is that it doesn't do it when the RPM's are going up but when they are coming down and when the engine is hot. I think it's either a cracked piston or a broken ring.
Tonight I will drop the pan and take a look.


I was so bummed out I shut the garage and went to bed.
Any thoughts.......