First off, your 2750 cfm electric fan will not move 2750 cfm thru a radiator, much less a 4-core aluminum radiator. The rating advertized is at 0" static. Your radiator will have about 1.8-2.2" of static pressure drop (assuming a 12FPI pitch). Depending on the brand of fan you are running, that will put you into about 1800-2200 CFM thru the radiator. The shroud you mentioned will make the most use of the air and ensures it moves thru the radiator. However with only a 1/2" off the core, you will not be utilizing the whole radiator. The best rule of thumb that exists is 1/2 the fan hub distance away minimum. This minimized the "air doughnut" effect. However if you have obstacles, you will need to compromise. If you move the fan off center a bit, can you get farther away?

The problems we face today can not be addressed at the same level of intelligence we were at when we created them - Albert Einstein Or with the same level of $ - Me