Oh heck....I know where the oil leak is! Its very common too. Its that darn oil filter housing....p.o.s. Its notorious for leaks. My suggestion, swap that housing out for the normal filter/water neck. It has to be from a mustang and not from just a 4.6l. The mustang water neck is correctly angled to the proper location.
The filter housing you have is serviceable, but im not 100% if premade gaskets are still made. Those housings were specific to 96Gt and Cobra's. Im thinking cobra got them for a couple of years. Im not even sure what its technical name is. Someone will post back, im sure. Common problem!