There are really not a lot of choices here. The objective is to move hot coolant into the intake hot spot.
If one taps into the lower half of the thermostat housing and into the block - there is no flow - that is until the thermostat opens. Trouble with this approach is that the intake 'hot spot' is constantly shedding heat. So this is not really an option.
In evaluating choices that actually produce flow there are some obvious limitations. The two ends of the heater loop are an obvious place to 'butt in' . . . but when plumbing through this route be aware that when the heater temperature control switch is set to cold there will no longer be flow through this path. When fitting tees also beware of this valve. Not to mention that both fittings are on the passenger side - making for long ugly plumbing.
Also, one is not so much concerned with robbing flow through the block - as one is concerned with impacting the opening and closing of the thermostat. Bringing cooled water into the chamber under the thermostat proper (via a second fitting) may create a hot condition in the block not visible to the thermostat as cooled water is flowing by . . .
Moreover, at operating temperature the thermostat is not constantly open. It cycles from closed to open to closed again. When closed the radiator is given time to do its job cooling . . .
