How much space is required between fan tips and shroud?

I was trying to install a shroud on my stock radiator in my 67 GTS. Didn't come with a shroud. For years I have run a 18" stainless flex fan. It does heat up some when you are at a light in summer. Lately I added water wetter which helped some. Thought a shroud would help more so I found a used stock shroud for the stock radiator. Only way to make if fit was with some trimming and I would have to mover the radiator a little which will only get me about 1/4" of clearance between the fan blades ends and the shroud. Not sure that's enough clearance for when the engine torques.