My Slant seems a little ill

You almost certainly have a blown head gasket. The factory cap is rated for 16lbs so there has to be some serious pressurization of the cooling system to force it into the overflow.

You can test the coolant for combustion gases but I'd go ahead and pull the head to look inside anyway. If it has a steel shim head gasket they can simply rust away and fail without being overheated. Good luck.