Where's the best place to tap into the 4L80 for a trans temp guage ?

On the left side where the gear selector is ,take it off and behind it is a plug
That's the spot ,some times you have to put a spacer on to have enough room.
That's between the plate and the trans.
David

My guage included a tapped block for the sender and compression fittings to install the sender in the hot line out of the transmission, before the coolers.