tune your car back to factory setting, use a hotter plug like OS #8, lean out slowly and read the engine temp again, your temp gauge might be faulty so do the water drop test on the heatsink and see how many seconds it takes to evaporate.

as for leaking, can you do a compression test? you can also go to auto part store to get some high temperature silicon (gasket maker) and put a very thin layer on the shim, then tighten up all screws evenly. Wait for couple hours then fire her up and see what happen.