I put a gauge made by Banks Engineering in our 4R100 transmission a couple years ago. They suggested putting the sending unit in the drain plug (if you have one). We didn't so Ford recommended putting a Tee in the line going to the transmission cooler (usually the top one). Then putting the sending unit in the vertical leg of the tee. It's not difficult. I got a 3/8" tee with anti vibration locks from McMaster Carr. (
www.mcmastercarr.com)
It took longer to run the wire from the gauge than it did to cut the line and install the tee & sending unit. Installing the gauge, sending unit, and wiring took about 2 hours. About half of that time was spent deciding where to install the gauge and how to run the wires.