OK bhere is the real deal.
Some time ago I pondered this same debacle. The viscous drive clutches from any Mfg are in my opinion, less than optimum. They work off airflow through the radiator (as you already know) to make the bimetal spring on the clutch activate the valve and engage the clutch..
Now as was mentioned at time you almost have to stop for the clutch to really take ahold. Poor system at best.
A full mechanical clutch would be far better, AHHHH HAAAA.
Now the T444E navistar in the IH trucks uses (on some models) a Hortor electric clutch.
This is the Powerstroke V8 incidentally.
The clutch screws onto the hub similarly to the late GM 6.5's but with a different thread size.
I bought one of these clutches. ($500) and then devised a way to get it onto the GM Pump.
The shaft size on the GM pump is the same as the 444E (stroke)
The secret is that the flange from the GM pump must be replaced with one from the stroke and then its off to the rodeo.
The Horton setup uses water temp to operate the clutch via a temp switch rather than air temp.
Simple and easy to set up but not cheep $$$$$$$$$$
New Pump
Flange from old stroke pump
Horton fan clutch (will accept 95 GM 6.5 metal fan)
Get flanges pressed on and off (pump rebuilders can do this easily)
For special applications this is sweet.
I made one up and have it sitting on the shelf. The only reason its not on the rig now is that I have yet to work out the final details of keeping the AC system working when the fan is off.
Need a port into the AC high side to activate the fan when pressure exceeds 250PSI
Also really need some air constantly for the AC condenser to work right.
For non AC truck it's easy.
Here is a link to the Horton catalogue
http://www.hortonww.com/catalog/index.asp
The unit you need is a 996124
Go to the catalogue and type this part number in the search box.
Also you will need to get another pump pulley and modify it in the lathe to fit the new hub pilot and also so it will bolt on.
Simple job though.
Here are some pix of my unit.
Hope this helps
MGW