Front end of the lower intake in the coolant passage.
Also, just a suggestion - run a 50/50 coolant/water mix. Straight water will eat up the water pump, rust out the heater core and destroy the head gasket at the coolant port between the lower intake and the heads.
1998 Roush: M112'd Aluminum block 4.6L 2V 6 speed track car
1999 SVT: Mostly stockish
1970 Mach 1: Resto-modded monster
2000 Roush #5004 - World's FIRST M112'd V6 Mustang. Sold, but not forgotten.