From my Ford manual it is 71-106 inch pounds. Unless my math is off that is 8.8 foot pounds.
If it has been together for a while, consider a new O-ring seal. Kind of expensive but will ensure a leak free result.
>>Excerpts from Ford service manual (2003 Mustang GT)
INSTALLATION
Clean sealing surface on water hose connection and on intake manifold (9424).
Position water thermostat, and water hose connection on the intake manifold. Install the two retaining bolts and tighten to 8- 12 Nm (71 - 106 lb. in). NOTE: To prevent incorrect installation of the water thermostat, the water hose connection on all engines contains an alignment slot which the water thermostat assembly fits it. Install the water thermostat with the bridge section in the water hose connection. Turn the water thermostat clockwise to lock it in position on the flats cast into the water hose connection.
Connect upper radiator hose to water hose connection. Move new hose clamp to water hose connection. Make sure clamp is between alignment marks on hose.
Fill cooling system with a 50/50 mixture of Premium Cooling System Fluid E2FZ-19549-AA or-B (in Oregon, F5FZ-19549-CC), recycled coolant or equivalent meeting Ford specification ESE-M97B44-A.
Start engine and check all hoses and connections for leaks.