This is probably a really basic question, but I figured better safe than sorry. If I use an electric fan on an aluminum radiator, what kind of gap should there be between the shroud and the fins? Could I just bolt up a piece of aluminum flat that covers the entire radiator cooling area- with the hole cut for the fan? Thanks for the help guys.

Most use no shroud and bolt the fan directly to the radiator. (fan style dependant)

I think no shroud would be better than bolting a plate to the back of the radiator. if the piece of aluminum sheet is up against the back of the radiator, it won't let air THROUGH the radiator very well. If you want the fan to pull air through the entire radiator I would think you need to set the fan back a little bit to provide some space between the backside of the radiator and the shroud, that way the air can go all the way through the radiator and then get pulled through the fan. this same project is on my list of to-do's for my car... get some sheet metal, tin snips, and pop rivets and make a custom shroud for an electric fan.

I have my radiators custom built by Portell's Radiator Shop in Desoto, Missouri. He installs dual fans with an aluminum shroud about 3/4 of an inch away from the core and basically sealed all around. This ensures that the air is pulled into the engine compartment and across the engine and not wasted flowing down the sides of the engine. I have never, I repeat, never had an overheating problem in the Camaro or the Corvette and anyone who owns a C-3 knows how hard they are to keep cool.

I do have to agree with Aaron that you should not place a plate against the fins.

Jim brings up another good point... if you can seal the shroud to the radiator, it forces the fan to pull ALL the air THROUGH the radiator and not pull air from around it, your cooling system will be a lot more efficient.