What type of thermostat is best?..

That is one important note. If the fan Blade is not partially out of the fan shroud, it cause turbulence(air move around/in shroud not thru and out eng compartment.) Stock fan and shroud......was already figure out for you.
If you are running a flex fan, you will have to figure this depth out. Usually a 1/3 of the blade out of the shroud.
the fan blade has to have a place to throw the air. and if its inside the shroud, that's were most of it goes and gets recycled, instead of OUT.