12" electric fan won't work properly, but a 16" does for me.......
The 350 SBC mounted too low in my 49 GMC to center a mechanical fan to the radiator so I went electric and I won't go back LOL.....
The radiator on a GMC is made a little differently than a same year AD Chevy in that it mounts a little further forward. This additional inch or so allowed me to make up a shroud and mount a 16" 2700 CFM electric fan to it. I use a fan sender mounted in the water jacket of the intake manifold that turns on at 200 degrees and turns off at 185 degrees
The fan hardly ever comes on when cruising below 60 MPH, avg'ing 190-195 degrees by being cooled by air flow through the radiator. It comes on when idling or when driving in heavy bumper to bumper traffic and the fan maintains the temps from 195 to 205. It's never gotten above 210 on 90 degree days.......
This is a pic of my fan/shroud setup:
Pic 1Pic 2Pic 3I saw a similar electric fan/shroud setup to this linked one on a Chevy AD at a car show.. It had just enough clearance and the owner said he had no cooling problems:
Link 
