MTBrian

Depending on which gauge inserts you get, you may need to remove the actual black numbers and needles OR you may be able to fish the needles through the larger holes.
There are some pretty detail instructions with pictures here: Mustang Gauge Installation
The installation of that bezel is a little involved. The plastic lense is super glued to the main black plastic bezel (see the pictures in the article). Installing the metal bezel requires seperating these plastic pieces, insterting the metal, and regluing them back together. This can be time consuming since you have to wait for the glue to dry and tedious because you don't want to crack, scratch, chip or otherwise damage the original plastic pieces in the seperation process.
The results are impressive however. The el-glo gauges and the bezel are decent looking enhancments.