[28 coupe]

I'm building a 39 chevy pickup. I rebuilt a 283 bored 30 over and a mild cam my question is how do I know what electric fan to use? I started to build a shroud but need to know the hole size to cut. I have 17 inches side to side and more than that top to bottom.I was thinking about a 16 in circle cut leaving a little on both sides for mounting.