If you go for an Autometer electric gauge you just need to drill and tap a 1/4-20 hole to thread the sender into. The autometer gauge won't work with the GM sender. I put mine directly into the top of my 3800 T-Stat housing.
There are also gauges that plug into the OBD-2 port ( Aero Force
) That can display tons of info.
1986 SE 3800SC/4t65eHD (email@example.com) I'll Sell it if you like
1985 GT 3800SC (swap in progress)