I had a '77 280SE that had a split exhaust manifold. It had the same problem but I couldn't find which section was leaking.
I finally used a short piece of garden hose and held one end close (but not on) the manifold area and the other to my ear. I moved it back and forth and quickly found the area of leakage.
I should also forewarn you to not put the hose right-up to your ear as it was VERY loud.
Give it a try.