Removing the fan is good

. We remove them from all of our race cars, and I don't own a street car that has a "engine driven" fan. At speed the fan doesn't do much more than use up HP. The air being forced through the grill will do all the cooling you need as long as your radiator is in good shape. Unfortunately, the original (new) GT radiator starts out marginal (fan or no fan) if you live in hot climates. (A trip through the Mojave Desert from LA to Las Vegas has "hurt" more than one "fun-run" by a group of Opel enthusiasts.)
A fan is REQUIRED for other than "at speed"... like around town driving. There just isn't any air being "forced" through the radiator. In all likelyhood, an electric fan will provide better cooling than the original fan. I don't know of any "kits" specifically for the Opel. I use a perma-cool 16" fan
http://www.perma-cool.com/Catalog/Cat_page18.html. I prefer to blow air through the radiator from the front (but pulling it from the back works too) because it is easiest to mount that way. Just make an "X" for the fan to mount to and bolt the "X" (with fan mounted) in front of the radiator. Take a look at the perma-cool dual fan set-up (
http://www.perma-cool.com/Catalog/Cat_page18-1.html) to get an idea of how to do it with a single fan... just don't block as much of the radiator as they do.
You don't have to do anything special to the pulley except you may need shorter bolts. Just remove the fan, and bolt the pulley back on.
Opel engines like to run around 180 degrees. So, anything that kicks the fan on at a little above that will be fine. I had a Fiero that didn't kick the fan on until 200+ degrees. For the Opel that would be bad. Perma-cool sells an "adjustable" kit that you could set for around 185 - 190 degrees. (You should have an 180 degree thermostat.)
Some "weasel words"... I started out by saying that a stock Opel GT radiator is marginal. I have only used electric fans on an Opel GT with a new heavy duty radiator (OGTS) or new aluminum racing radiator. Although I think an electric fan will work better than the stock fan, you do it at your own risk on a stock... old radiator.
