An electric fan in front of the radiator will give you the most room to move the engine forward if your application needs it, it is also the least efficient setup. A thin electric fan puller (behind radiator) would be next best for clearance and a lot better cooling. Next clearance wise would be a flex fan then finally a factory style clutch fan. The clutch fan will move the most air and cool better than any of the prior setups. Another concern is that all of these must be properly shrouded or you will suffer a huge loss of efficiency...