I'm using 5" speedo and tach.
Then the 2 5/8 for the oil pressure, water temp, fuel level, volt meter.
You don't need to buy any special sending units.
However, I did mount my oil pressure and water temp sending units with the kit that JCI sells. His kit is called the Vital Liquids Set.
http://www.brokenkitty.com/zcar/zcar.htm
I found the stock oil pressure and water temp sending units didn't go well with the JCI adaptors, but the autometer ones fit PERFECT !
As for the autometer water temp sending unit, you can use the fittings in JCI's set, just have to take the brass fittings apart and combine them with the autometer fittings