Dude, you gotta let the engine get warmer than that. You're killing your cylinder walls. You're also not getting the oil hot enough to boil out the condensation which will then combine with combustion by-products that make their way into the oil. This will make the oil extremely acidic w/o frequent changes (100-500 mile intervals). Unless you had the engine built as an Alcohol engine, i.e. intended to run at that temp with frequent oil replacements. It needs to get to 180-190. Yea, it may not make as much power at those temps, but it will live a heck of a lot longer. And the dang heater will work during those cold snaps.
Off soap-box.......
JCW sells an adjustable electric fan t-stat. I've used one for years w/ no problems.