there are a lot of different types of white face gauges. if you check out the how-to on mustangworld.com, they use mac gauges that are basically big stickers. i think the ones tango got are indiglo overlays. they lay on the original face, sliding under the needles while they're still on. they don't rely on the back-lighting of the stock gauges and don't have an adhesive. i used double sided tape, but don't know if it was necessary. it was mostly because i didn't want to have to take them off if the overlay didn't lie flat.
98GTBlk- I installed mine myself, and trust me, im no professional.