I don't know what pistons are in your engine, but if they are the typical Ford cast style then you could have cracked or completely lost a skirt. This was a very common problem at one time and the reason everyone used forged pistons in rebuilds. You can pull one plug wire at a time while the engine is idling and see if the noise goes away or changes on one specific cylinder. Take away the fire and you take away the pressure that makes the knock. This stupid little trick wont tell you if it is a piston skirt, wrist pin or bearing, but it will locate it if it is in the bottom end. If the noise doesn't change then it's a top end issue. Sharp clacking is typically a piston. A heavier sounding deep "thunk" is usually more like a rod bearing noise. There are of course other things that can make noise in an engine. Piston skirts wont cause an
oil pressure loss or drop, a rod bearing failure most certainly will. Best of luck and keep us posted.
Edit: I have had a couple small block Fords make the piston skirt noise that I would have bet money on and it turned out to be a rod bearing or once even a cracked flexplate. Also, if someone swapped over to timing gears, I have heard them develop one hell of a knock too.