Vintage Air sells 6 blade solid steel fans that move a ton of air, they mount with 50% of the blade in front and 50% behind the mounting pad. Most fans are off set, so this may help with your limited space requirements. I have the 17" version which cools my 250 and A/C just fine. I also have a fan not centered on the core in my '37 truck.
A pusher fan will just block air when driving and will only cool the portion it's blowing through if mounted directly to the radiator. Ideally you want to pull air through a core with a shroud big enough for air to pass through the whole core and make the corner turns to exit by way of the fan opening. If the shroud is too close, the corners won't cool due to the restrictions of the tight shroud. The shroud should also be sealed to the core so all air the fan pulls, must come through the core of the radiator.
With out a shroud your next best option is to add more aggressive fan blades or a faster rpm of the water pump pulley.
You could try adding a spacer and moving the fan closer to the core if you have room, or try turning up the rpm at idle.
