cooling my slant 6

I might add, if you don't keep topping off the coolant, it will just stay at a constant level, well below the neck. When you fill it all the way to the neck when cold, it naturally overflows as the fluid warms up and expands, which makes the level look "low" again when you check it cold. So just leave it "low". This is assuming you don't have a leak somewhere.

Anyway, if you don't see a cloud of steam, you're probably not overheating.