My radiator is a 3-1/4" thick, dual core aluminum unit. Standard mustang width and height.
I only have around 3-3/4" or so from the radiator to the nub on the water pump. The Derale 16926 fan is only 3-1/4" thick, so it gives me about a half inch of clearance between the water pump and fan.
With the power and amp draw of this fan, I recommend using 10 gauge power wire, a relay, and very good, preferably soldered connections.
As for amp draw, I HIGHLY recommend the 3G alternator upgrade! These came on early 90's model Taurus, etc. They are EASILY adapted to older cars, and put out 130 amps... even at low rpm. I got one out of the boneyard back in the day for $30, and am still using it. Runs EVERYTHING... electric fan, water pump, fuel pump, headlights, etc... even at an idle without discharging. It's a wondrous piece of engineering. lol If you get one from the boneyard, it has a metric tightening bolt. You'll probably want to get that with it. Also, grabbing the wiring pigtail makes life easier as well.
p.s. It sounds like you have some bad measurements... Either that, or you are leaving the spacer in place while taking them. Remove the spacer off the front of the water pump before getting the tape measure out!