Here are a few tips in installing a dash in a GT when the windshield is already in the car.
First off you need to have the flexible screwdriver attachment for a drill. It looks like this
And you will need a longer (2 inch) phillips bit instead of the shorter ones shown in the photo above.
You will need a drill (preferably cordless).
To install the dash:
1) Fit dash in place in car.
2) Affix the dash using the 3 screws on each side of the dash (the screws by the doors) using a regular phillips bit screwdriver (by hand)
3) Using the flex screwdriver, attach the screw on the top of the dash on the drivers side.
(To help line the screw up you can look through the opening for the instrument cluster and see the hole the screw is supposed to go into)
4) Affix the passenger side outer screw on the top of the dash. This one will be the hardest to line up.
5) Affix the middle screws on the top of the dash
6) Affix remaining screws on the lower middle of the dash.
7) Affix the heater control
8) Reattach steering column
(optional if you want to wait until after the instrument cluster is installed which is recommended)
To start the screws in the middle on the top, put a little temporary adhesive on the tip of your finger and stick the screw onto your finger to guide it into the hole in the dash.
If the outer screws are tightened properly, the middle holes will line up fine. The middle screws are harder to get into the dash holes but much easier to get to with the screwdriver bit.
Using the steps above, it took less than 45 minutes to install the dash. That included the time it took me to find my can of spray adhesive.
You can also try to install the dash with the screws already in the holes on the top of the dash but I found it easier to get the two outer ones by placing the screw on the screwdriver bit and guiding it in. It's a bear to try to get the screwdriver bit into the screw head when the screw is in the dash. This applies to the outer top ones only. The rest are easier to get to while in the dash holes.
Hope this helps.