First off, you have to find EXACTLY where the leak is coming from....
Yes, the uppper plenum is known to have problems, and if it is leaking, has to be replaced......
HOWEVER, that is usually an INTERNAL leak, not an EXTERNAL one.....
From your description, look at the center of the lower intake manifold, on the passenger side.....there is a 90 degree plastic elbow coming out of it and going into the tensioner assembly......that elbow cracks/breaks just inside the LIM....I think that is your problem.....a pressure test will confirm this with a coolant pressure tester.....
To replace this, you remove the belt, drain the radiator, and remove the whole tensioner assembly(there is another elbow on the bottom that will have to be replaced too).....