Electric fans - how much cooling is necessary?

[Number21]

My '73 350/350 is getting a whopping 9mpg. That's only 180 miles/tank! Part of the cause of this is that I don't have a fan clutch. I don't have a fan shroud either, and I have a new radiator, so I'd like to swap in some electric fans.

There are so many different fan options to choose from...just how much cooling do I need? I was thinking a combination of a 16" fan rated at 2500 CFM and an 11" fan rated at 1000 CFM. That way I have a backup if one fails, I can use both at once, or just one or the other. I can buy both locally for about $75. Are these fans going to be powerful enough? Do I really need to try and make some kind of fan shroud? It seems to be doing quite well now without one...