I use an electric fan off of a 90' V6 Taurus. It keeps my car plenty cool in the middle of summer with the AC blowing and stuck in traffic...It has a high and low input and I run it on low about 10 months of the year...This is on a black auto coupe with a 3 core brass cooper radiator. I would recomend getting fan off of a Lincoln Mark VIII, Late 80's early 90's V6 Tarus, Or a 94' up V8 mustang...These are basically the same fan with slightly different shroud designs. You'll have to do some triming on all of these and you will need to have a 130amp alternator. These flow huge ammounts of air but a lot of amps too. Origianlly I had a 16" Flex a lite fan, but it didn't cut it, especially wit the AC...I paid $110 for the flex a lite, and I got this one from a salvage yard with a 60 day return for $30.. do it right the first time, it sucks doing it twice and driving around in august without AC...The mark VIII fans are a really popular choice but are hard to find and are more expensive, the others are more common and less expensive...
If you're running a 31" radiator you only have about a 25" wide core so what ever fan you find needs to fit that...As mentioned above it is more important to find something that covers as much surface area of the core as possible, that is more important than the CFM rating...
I do prefer the reliablity of a mechanical fan, but its nice being able to cool down quickly while sitting in the pits...
Mark VIII install
good luck, hope this helps