Jim,
You can put the gauge in as long as your arm is long and being triple jointed would help! By the time you go through all that it is easy to just grin and bear it! Don't take the seat out, just let both feet dangle over the top of the seat.
First of all cut the hole a little smaller than needed, Lift up the covering of the dash and then file away until you get the dash to fit. Now you can take the extra material around the hole and fold it in place. You can use spray glue on the back but if you took your time and cut it right - shouldn't need any glue.
Remember do NOT go any higher than CR's did with the gauges or you will hit the bar for the dash support.
Auto body store: They make a two part glue for fiberglass to steel----very pricey and wait till you go to buy the tool to use it!
Product is SEM or 3M works great!
DV