fitting size??

[70c10guy]

I have a mechanical three gauge panel by moon in my truck. I installed it and hooked oil, and volts up. Still haven't hooked water temp up. There is a fitting on the end up the line that screws into the block. I need the size of thefitting on the gauge side before the sender. Now I have a 5.3 going in it now and I know the size of the hole in the head is 12x1.5... I read the thread pitch on the fitting that's on the line and it's also 1.5... I do not know the diameter?? I obviously need an adapter fitting. Male 12x1.5 to female ____x1.5 for the other fitting to screw into. I'm stuck!!