[FWB]

using Patrick's pic as a guide i use the port closest to the distributor. that port will pick up the temp from crossover between the two heads.

and yes the thread is NPT, i think if i remember its 1/4 NPT if you buy a tap be sure its not just a carbon steel tap or you may break it off. make sure the tap package or the seller shows it to be HSS high speed steel.
use tapping fluid for lubrication or liquid soap if none is available. if you have any tube lying around use it as a sleeve for the tap to insure its going in straight...