Vacuum advance can help keep it cooler a highway speeds (more advance can increase efficiency and reduce heat)... but probably need to tune it carefully with high CR for pump gas. Don't think it will help with the stumble though... when you step on the gas, manifold vacuum drops and vacuum advance should be basically inoperative. Also, lean=hot to a point so verifying AF ratios sounds like a good idea.

Still, it sounds like a strong cooling system... 4 core radiator ect. Probably should keep it cool regardless. Are all cooling components in known good shape?