Pusher vs Puller Electric Fan
A puller will be slightly more efficient than a pusher because it masks the radiator core. Probably not a big deal with a properly sized fan.
You should chose electric or mechanical, not both. Also avoid flex fans like the plague. They are noisy, inefficient, and may hurt you at high speeds. An OEM style clutch fan is still your best bet if you have room.
If you can mount the Spal unit you have now as a pusher, give it a shot. It's already paid for, right? If it doesn't work, try the MP clutch fan.