on my 98GT I did not need the nipple. just 2-o-rings.
Don't forget the web site for picks and instruction. www.unitedmajority.com
there is a pic of the sensor tap with details on
where to buy it. I have not tapped a plastic crossover, this does
sound risky. You could probably find a manifold for (alum. crossover) $160 on e-bay look for a seller called the "stangstore" or try www.mustangtuning.com @ $198.00 for a manifold.
The tee fitting should work but the reaction time will be slow due to the sensor not sitting in the flow path.