I would look for a fan from a motorcycle. It would be better suited for the job than a computer fan.
What are you trying to accomplish? The only reason I would install a fan is if I was doing ALOT of slow speed running and idling. At high speeds the fan will only restrict air flow and hinder cooling.
My 81' Invader stays much cooler than my 79', if you have a 78-79 Invader, you may want to consider upgrading to the larger heat exchanger.